Fix or remove gstreamer generic image extraction path

We have a '90-gstreamer-image-generic.rule' extract rule which can fail in damaging ways.

The underlying cause is https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/691 -- a GstDiscoverer misfeature where it decides that unknown files must be MP3s and feeds large amounts of data into an MPEG decoder, which causes very high CPU and IO usage and sometimes causes Linux to hang altogether.

The two recent issues are:

• tracker#158
• !120 (merged)

This extractor is of dubious utility anyway. In the best case it'll only manage to return the width and height of the image, which is not very useful information.

I propose for 3.0 we remove this extractor as I don't see a use case.

Another option, which would go a long way to reduce the risk of causing system lockups, would be reducing the timeout we pass to gst_discoverer_new() from 5 seconds to 1 second.