debian/postinst: Remove gedit as a gnome-text-editor alternative

Also make the removal in debian/prerm conditional, since the alternative may no longer exist.

I tested a local build of this in a few different scenarios in my sid VM, and it seems to work well and as expected.

requested review from @jbicha

changed the description

added 9 commits

• 6539adfc...a64d8283 - 8 commits from branch gnome-team:debian/latest
• cd27c7b7 - debian/postinst: Remove gedit as a gnome-text-editor alternative

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merged