MIR: Remove 2 empty non-reverted directories form SVN.

The revert commit r238007 didn't delete the old 'MIR' directories
in 'include/llvm/CodeGen' and 'lib/CodeGen'. Since the resubmitted
commit r238341 uses the 'MIRParser' directories instead of 'MIR'
this commit deletes the unused directories.

llvm-svn: 238352