Draft: Remove obsolete gtk-update-icon-cache package, now built by gtk4

We can't push this yet because the new version of gtk4 that builds the gtk-update-icon-cache binary package is still in Experimental.

Even then, maybe we want to tweak this to keep building gtk-update-icon-cache on most Debian ports since gtk4 isn't built there currently.

I noticed this issue today because Launchpad does not allow uploads of superseded binary packages but I believe Debian doesn't have a block like that.