Repository: yt-dlp
Issue: `-x` downloads video too if combined with `-k`
Labels: bug,triage

### Checklist

- [X] I'm reporting a bug unrelated to a specific site
- [X] I've verified that I'm running yt-dlp version **2022.01.21**. ([update instructions](https://github.com/yt-dlp/yt-dlp#update))
- [X] I've checked that all provided URLs are alive and playable in a browser
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/ytdl-org/youtube-dl#video-url-contains-an-ampersand-and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues including closed ones. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)

### Description

It is widely known that `-x` is used for extracting audio, but when `-k` is used with it, it downloads video format too.
This theoretically happens with any video where `bv` and `ba` match to any formats, like YouTube.
I don't think this behavior is correct, since `-x` stands for `--extract-audio` and `-k` doesn't mention about format selection.
(Using `-f ba/b` can prevent this if you need to skip video)

Everything except that is normal and expected.

### Verbose log

```shell
$ yt-dlp --verbose -x https://www.youtube.com/watch?v=5cWZhw3rYM0 
[debug] Command-line config: ['--verbose', '-x', 'https://www.youtube.com/watch?v=5cWZhw3rYM0']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, err utf-8, pref UTF-8
[debug] yt-dlp version 2022.01.21 [f20d607b0]
[debug] Python version 3.9.10 (CPython 64bit) - Linux-5.13.0-25-generic-x86_64-with-glibc2.34
[debug] exe versions: ffmpeg 4.4 (setts), ffprobe 4.4
[debug] Optional libraries: Crypto, sqlite
[debug] Proxy map: {}
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=5cWZhw3rYM0
[youtube] 5cWZhw3rYM0: Downloading webpage
[youtube] 5cWZhw3rYM0: Downloading android player API JSON
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, quality, res, fps, hdr:12(7), source, vcodec:vp9.2(10), acodec, lang, proto, filesize, fs_approx, tbr, vbr, abr, asr, vext, aext, hasaud, id
[info] 5cWZhw3rYM0: Downloading 1 format(s): 251
[debug] Invoking downloader on "https://rr2---sn-oguesn6d.googlevideo.com/videoplayback?expire=1643673295&ei=byL4YZu5LLnVs8IPpOei2Ag&ip=2401%3Ac080%3A1000%3A4d16%3A5400%3A3ff%3Afe8b%3Ad04e&id=o-AN0VtmnS8x343PJkq7-TwQYM_eU5IUZpQDfhCbdV3cWM&itag=251&source=youtube&requiressl=yes&mh=Ga&mm=31%2C29&mn=sn-oguesn6d%2Csn-oguelnl7&ms=au%2Crdu&mv=m&mvi=2&pl=48&initcwndbps=693750&vprv=1&mime=audio%2Fwebm&gir=yes&clen=9159125&dur=568.921&lmt=1580640686955750&mt=1643651332&fvip=2&keepalive=yes&fexp=24001373%2C24007246&c=ANDROID&txp=5531432&sparams=expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRQIgEj4tWCf-ZC5VfIuekNr1c6Wj47IEiSzN0WkPSAIMUyQCIQDHuUY7xdroTwE77WF0Yae45G8YL2sXt_SYnLBGi-TmCw%3D%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRQIhAJKWf_peFFzIV1Xs9KxhjOdGaX0g2uQINixMiH-5TbHvAiBZ-B99UOTT4pkMA-k_c2cHdF8BpHMug7-JT3ebkhMw_g%3D%3D"
[download] Destination: 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm
[download] 100% of 8.73MiB in 00:00
[debug] ffmpeg command line: ffprobe -show_streams 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm'
[ExtractAudio] Destination: 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].opus
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm' -vn -acodec copy -movflags +faststart 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].opus'
Deleting original file 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm (pass -k to keep)

$ yt-dlp --verbose -k -x https://www.youtube.com/watch?v=5cWZhw3rYM0 
[debug] Command-line config: ['--verbose', '-k', '-x', 'https://www.youtube.com/watch?v=5cWZhw3rYM0']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, err utf-8, pref UTF-8
[debug] yt-dlp version 2022.01.21 [f20d607b0]
[debug] Python version 3.9.10 (CPython 64bit) - Linux-5.13.0-25-generic-x86_64-with-glibc2.34
[debug] exe versions: ffmpeg 4.4 (setts), ffprobe 4.4
[debug] Optional libraries: Crypto, sqlite
[debug] Proxy map: {}
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=5cWZhw3rYM0
[youtube] 5cWZhw3rYM0: Downloading webpage
[youtube] 5cWZhw3rYM0: Downloading android player API JSON
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, quality, res, fps, hdr:12(7), source, vcodec:vp9.2(10), acodec, lang, proto, filesize, fs_approx, tbr, vbr, abr, asr, vext, aext, hasaud, id
[debug] Default format spec: bestvideo*+bestaudio/best
[info] 5cWZhw3rYM0: Downloading 1 format(s): 248+251
[debug] Invoking downloader on "https://rr2---sn-oguesn6d.googlevideo.com/videoplayback?expire=1643673320&ei=iCL4YbXMC5iU1d8PjLuEuAc&ip=2401%3Ac080%3A1000%3A4d16%3A5400%3A3ff%3Afe8b%3Ad04e&id=o-AJX4MyZtcoRuJgJyJagnZCtXMiE5tdV9rbV7ET0EFg17&itag=248&source=youtube&requiressl=yes&mh=Ga&mm=31%2C29&mn=sn-oguesn6d%2Csn-oguelnl7&ms=au%2Crdu&mv=m&mvi=2&pl=48&initcwndbps=693750&vprv=1&mime=video%2Fwebm&gir=yes&clen=84618307&dur=568.901&lmt=1580641501397302&mt=1643651332&fvip=2&keepalive=yes&fexp=24001373%2C24007246&c=ANDROID&txp=5535432&sparams=expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRgIhALSbp8GYMwuDzWruWYeRiEM0bDNJ4S0oEtYtmHmN3OqdAiEA35AZrH-lWBgjwStyd6VgzS04yNFaw3ByOusZdowqzXY%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgfXl1aI2-Nls-H61n3y4ho0QsFGiqzv4MhnVr5xXGRsICIA5zdO6SkT60CO476CD5PR3c40ebmD1qvxd96Ahpspnt"[download] Destination: 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f248.webm
[download] 100% of 80.70MiB in 00:00
[debug] Invoking downloader on "https://rr2---sn-oguesn6d.googlevideo.com/videoplayback?expire=1643673320&ei=iCL4YbXMC5iU1d8PjLuEuAc&ip=2401%3Ac080%3A1000%3A4d16%3A5400%3A3ff%3Afe8b%3Ad04e&id=o-AJX4MyZtcoRuJgJyJagnZCtXMiE5tdV9rbV7ET0EFg17&itag=251&source=youtube&requiressl=yes&mh=Ga&mm=31%2C29&mn=sn-oguesn6d%2Csn-oguelnl7&ms=au%2Crdu&mv=m&mvi=2&pl=48&initcwndbps=693750&vprv=1&mime=audio%2Fwebm&gir=yes&clen=9159125&dur=568.921&lmt=1580640686955750&mt=1643651332&fvip=2&keepalive=yes&fexp=24001373%2C24007246&c=ANDROID&txp=5531432&sparams=expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRgIhAKhXofgcXVGcegsismq5EjMKglQwj40T0loJNoMeSaMjAiEAsx-ScEfavtYdpwyf6cIr6-aj_F_lNHaE-fW0KEUPMDE%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgfXl1aI2-Nls-H61n3y4ho0QsFGiqzv4MhnVr5xXGRsICIA5zdO6SkT60CO476CD5PR3c40ebmD1qvxd96Ahpspnt"
[download] Destination: 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f251.webm
[download] 100% of 8.73MiB in 00:00
[Merger] Merging formats into "【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f248.webm' -i 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f251.webm' -c copy -map 0:v:0 -map 1:a:0 -movflags +faststart 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].temp.webm'
[debug] ffmpeg command line: ffprobe -show_streams 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm'
[ExtractAudio] Destination: 【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].opus
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm' -vn -acodec copy -movflags +faststart 'file:【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].opus'

$ ls *5cW*
'【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f248.webm'
'【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].f251.webm'
'【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].opus'
'【公式】サンドウィッチマン コント【パソコン教室】2015年 [5cWZhw3rYM0].webm'
```