Drop GTK2 support in favor of GTK3

[Caolán McNamara <caolanm@xxxxxxxxxx>]

https://gerrit.libreoffice.org/#/c/77405/ is my proposal to drop
LibreOffice's Gtk2 support in favor of the Gtk3 vclplug for LibreOffice
6.4. Gtk3 has been the default for quite a while and f.e. in Fedora we
dropped building Gtk2 since Fedora 30. This simplifies the ifdef maze
and drops the include gtk2 source from gtk3 versions hackery

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