scons: prune unused Makefile.sources

Fixes: 2e92d338 ("scons: Prune out unnecessary targets.")
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed by: Jose Fonseca <jfonseca@vmware.com>

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <mesa/mesa!4373>
Part-of: <mesa/mesa!4373>
47 jobs for !4373 with cleanup-scons-makefiles-sources in 26 minutes and 59 seconds (queued for 13 seconds)
merge request