help-mpi-api.txt: remove stale opal_show_help() message

Thankfully, we no longer have a private info key prefix namespace.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

ompi/mpi/help-mpi-api.txt

@@ -2,7 +2,7 @@
 #
 # Copyright (c) 2006      High Performance Computing Center Stuttgart,
 #                         University of Stuttgart.  All rights reserved.
-# Copyright (c) 2006-2015 Cisco Systems, Inc.  All rights reserved.
+# Copyright (c) 2006-2022 Cisco Systems, Inc.  All rights reserved.
 # Copyright (c) 2018 IBM Corporation. All rights reserved.
 # Copyright (c) 2020      The University of Tennessee and The University
 #                         of Tennessee Research Foundation.  All rights
@@ -30,8 +30,3 @@ this environment.
 
   MPI function: %s
   Reason:       %s
-[info-set-with-reserved-prefix]
-Comments
-MPI_Info_set warning, key is using a reserved prefix.
-  Key:             %s
-  Reserved prefix: %s