third_party cleanup

Remove unused old mpc tarball.
Extract the expat license file from the tarball to match existing practice.

BUG= none
TEST= none
R=bradnelson@google.com

Review URL: https://codereview.chromium.org/134163004

git-svn-id: svn://svn.chromium.org/native_client/trunk/src/third_party@12630 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2