Deprecate ENABLE_COLL_FT_RET

This environment variable is no longer necessary now that FT and NBC will play
well together. We'll keep this one around for one more version, then delete it
in MPICH-3.2.
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

src/util/param/params.yml

@@ -319,6 +319,7 @@ parameters:
       default     : true
       class       : device
       description : >-
+        DEPRECATED! Will be removed in MPICH-3.2
         Collectives called on a communicator with a failed process
         should not hang, however the result of the operation may be
         invalid even though the function returns MPI_SUCCESS.  This