firefox-bin: remove libgnome dependency

It is deprecated and has been removed ages ago:

https://bugzilla.mozilla.org/show_bug.cgi?id=694570

It persists in the crash reporter for some reason but it is optional there:

https://github.com/mozilla/gecko-dev/blob/f66f5a235e7d74c29b951316f73001126a056734/toolkit/crashreporter/client/crashreporter_linux.cpp#L366-L370