Removed mrmpi ThirdParty, which was not used.

mrmpi (map-reduce mpi) was not used in VTK. Removed.

Change-Id: I239717a7348ea9ab43e7404f2a45c8a46e0b329b
Merge topic 'removed-mrmpi' into master

4bbd890a Removed mrmpi ThirdParty, which was not used.