README: Remove MXM from MTL list

The MXM MTL has been deleted from Open MPI; no need to include it
in the README anymore.

Signed-off-by: Dennis Field <fury@xibase.com>

README

@@ -507,7 +507,6 @@ MPI Functionality and Features
   - yalla
 
   (1) The cm PML and the following MTLs support MPI_THREAD_MULTIPLE:
-      - MXM
       - ofi (Libfabric)
       - portals4