[2.x] Remove generic (GStreamer) image extraction rule

See tracker#95 (closed) for an endless list of people reporting tracker-extract locking up their PC's, wasting their time and ours as we try and debug the problem. In every case so far the cause has been src/tracker-extract/90-gstreamer-image-generic.rule triggering https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/691

It seems we have no option but to remove this codepath. @carlosg - do you think we need to make a 2.4 release series for that, or if it can be snuck into a 2.3.x release?