Stop distributing CoqIDE

CoqIDE currently requires gtksourceview2, which has been removed from
Debian (see https://bugs.debian.org/885677). Upstream has an active
pull request to update to gtksourceview3
(https://github.com/coq/coq/pull/9279), and it looks like Debian may
ship gtksourceview2 with buster anyway, so this is likely to be a
temporary change.