propose-library-branches: remove unused --all option

The --all option was copypasted from propose-final-releases but
ignored afterwards.

Change-Id: I1023c659506b12d833d856f16b604c8ba24c11f5

openstack_releases/cmds/propose_library_branches.py

@@ -33,12 +33,6 @@ def main():
         action='store_false',
         help='do not remove temporary files',
     )
-    parser.add_argument(
-        '--all',
-        default=False,
-        action='store_true',
-        help='process all deliverables, including release:cycle-trailing',
-    )
     parser.add_argument(
         '--verbose', '-v',
         action='store_true',