Bug#853910: meta-gnome3: consider dropping empathy dependency

[Jeremy Bicha]

> even though it's still in the upstream modulesets.

Not any more:

https://git.gnome.org/browse/jhbuild/commit/modulesets?id=f981a0b75d

Unless someone ports empathy to WebKit2, I think it's likely it will
be dropped for buster. Not installing it by default (not even
recommending it from 'gnome') seems a reasonable step and matches what
other distros do. (It is no longer installed by default as of Ubuntu
16.04 LTS, Ubuntu GNOME 16.10, Fedora, etc.)

Thanks,
Jeremy Bicha