Fixes #4812: Remove usnic warning upon cleanup

From provided patch
Fixes #4813: usnic: update help messages

From provided patch - note hunk offset reports:
patching file ompi/mca/btl/usnic/help-mpi-btl-usnic.txt
Hunk #1 succeeded at 189 (offset 32 lines).
Hunk #2 succeeded at 207 (offset 32 lines).
Hunk #3 succeeded at 234 (offset 32 lines).
Hunk #4 succeeded at 257 (offset 32 lines).