scons: Prune out unnecessary targets.

This prunes out all targets except libgl-gdi, libgl-xlib, and svga, as
suggested by Marek Olšák.

libgl-xlib will be remove once I have had time to confirm no automated
tests we have rely upon it.

There are also a bunch of Makefile.sources which become orphaned as
result, that are not taken care of in this change.

v2: Prune remainders of swr support.

Tested-by: Marge Bot <mesa/mesa!4348>
Part-of: <mesa/mesa!4348>
49 jobs for !4348 with scons-prune in 26 minutes and 37 seconds (queued for 5 seconds)
merge request