Remove dead image hosts

Issue #1582.

The websites for eho.st, picsarus, and memefive no longer exist, so I don't think there's any reason to keep them.

Should we remove picshd? Although [no images have been posted to reddit from there in a year,](https://www.reddit.com/search?q=site%3Apicshd&sort=new) the old images still work.