Remove src/mcas

Remove src/mcas

This lock-free library toolkit is intriguing and may be the subject
of future work, but such development will occur on the master branch,
and these files are just clutter on openafs-stable-1_8_x.  Remove
them to give the tree a more clean start.

Remove src/mcas and stop mentioning it in SOURCE-MAP; don't reference
it in the rpctests, either.

Change-Id: I21b1b6b64a709fe40aa53aaf3470d128c0dc2f86
Reviewed-on: https://gerrit.openafs.org/12682
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>