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

[intrigeri]

olly just nicely explained me on IRC a few things about the
autoremoval machinery and how it affects removing from testing
libgtk2-perl and its reverse-dependencies:

 - A set of RC buggy packages that have no reverse-deps outside of
   this set, should all be autoremoved together at some point, as long
   as they're not key packages, which are immune to autoremoval:
   https://udd.debian.org/cgi-bin/key_packages.yaml.cgi

 - libgtk2-perl is on the list of key packages because it's installed
   on many machines ("popcon"). It's the last we can remove because
   all other packages that I've made RC-buggy today are
   reverse-dependencies of it.

 - Once everything else is gone, we'll need to ask for libgtk2-perl
   to lose its "key package" status and be removed from testing.