Remove unused code when download from youtube.

We now convert transcripts from 1.0 speed to other speeds on front-end.
Previously we downloaded transcripts for all youtube speeds, and did conversion from existing speeds to other speeds. 
But the only right and most used in courseware case is when youtube 1.0 speed has transcripts. Most courses are set-up with youtube id for 1.0 speed and transcripts for this speed. Basically course authors always care about 1.0 transcripts, and do not think about other speed transcripts any more, as they are converted from 1.0 transcripts automatically. Code that existed became redundant in terms of supporting corner cases that were never used.

@polesye @olmar please review.
`good_youtube_subs` -> `good_youtube_sub`