deps: Drop gtk+-3 to suggests for gst-plugins-base

These GStreamer plugins build examples with GTK+ 3, but those are non-
essential, so we should avoid building GTK+ 3 if not needed or wanted,
especially because gst-plugins-base can now be pulled in by GTK+ 4…

https://bugzilla.gnome.org/show_bug.cgi?id=794820