Bug#885143: libreoffice-gtk2: Drop gtk2 support

[Jeremy Bicha <jbicha@debian.org>]

Package: libreoffice-gtk2
Version: 1:6.0.0~rc1-1
Tags: buster sid

Please consider dropping gtk2 support even if upstream continues to
provide a gtk2 compile option.

While it's impractical in 2018 to have a Debian desktop without gtk3
installed (unless you want an obscure web browser), it will be
possible to have a desktop without gtk2 installed. And if gtk3 is
required anyway, why try to use gtk2?

libreoffice-gtk2 is currently used by default in the Debian Tasks for
Xfce, LXQt and LXDE. That may have made sense for Stretch because
Stretch shipped LibreOffice 5.2 (the first version where the gtk3
version was ready) but I don't think it makes sense for Buster.

There's a support burden to provide the gtk2 version that may be
unjustified if nearly everyone is using the gtk3 version.

Thanks,
Jeremy Bicha