scons: prune unused Makefile.sources

Merged Eric Engestrom requested to merge eric/mesa:cleanup-scons-makefiles-sources into master

@jrfonseca there were only two Makefile.sources that were actually orphaned, the rest are still used by Android.

Fixes: 2e92d338 ("scons: Prune out unnecessary targets.")

/cc @tpalli I'm sending this to the Intel CI, but just in case it doesn't cover everything on dev builds, could you have a look and confirm there isn't something somewhere that needs these two files?

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