Some people need to avoid inclusion of GPL code for their use cases and thus
get easily spooked by GPL license headers. This code is actually licensed
under different licenses, only one of which is GPL, and it's already possible
to just upgrade from LGPL to GPL anyway so having the GPL listed explicitly
as one of the choices doesn't really add anything. So remove GPL from the list
and also add SPDX license identifiers while we're at it.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2539>
