net-libs/gtk-vnc-0.7.2: remove gtk2 support, simplify ebuild

net-libs/gtk-vnc-0.7.2: remove gtk2 support, simplify ebuild

Remove gtk2 library, which isn't used by anything in tree anymore.
This also removes IUSE=gtk3, which should be fine as all consumers
were adjusted to require gtk3(+); only an older gnome-boxes version
doesn't yet, but matching ~arch version does have it, and it'll
be stabilized before gtk-vnc-0.7.2 is, so should be fine.

This greatly simplifies the ebuild because we don't need to use
multibuild.eclass anymore. Additionally it fixes the lack of
installed docs (NEWS, README, etc), which were broken due to
multibuild usage - after its removal, the default einstalldocs
without DOCS set kicks in and installs exactly what we need.

Also remove VALA_USE_DEPEND="vapigen", as vala versions that had
the USE flag are long gone (eclass generates an unnecessary
[vapigen(+)] USE dep if VALA_USE_DEPEND is set like this).

Package-Manager: Portage-2.3.28, Repoman-2.3.9