Bug#912860: Don't ship libgtk2-perl in Bullseye

[intrigeri]

I've filed bug reports against all reverse dependencies (normal
severity for now), tracked using the gtk2-removal usertag:

https://udd.debian.org/cgi-bin/bts-usertags.cgi?tag=gtk2-removal&user=debian-perl%40lists.debian.org

All reverse dependencies are leaf packages. The vast majority are
either dead upstream, or orphaned in Debian, or de facto unmaintained…
or, pretty often, some combination thereof.