thirdpartymirrors: Remove github

thirdpartymirrors: Remove github

GitHub has deactivated the old downloads API in ~2013.  While the URLs
seem to work for previously uploaded archives, no new archives can
be uploaded this way and old archive URLs are not really visible
anywhere.  Furthermore, the mirror entry existed only to provide hacky
short-cut in download access which stopped working anyway.

All of the remaining mirror://github uses have been inlined, so just
remove the entry.

Closes: https://bugs.gentoo.org/481382
Signed-off-by: Michał Górny <mgorny@gentoo.org>