flux-hostlist: allow --nth to used with --expand and --count

Problem: The flux-hostlist `-n, --nth` option no longer needs to be
in the mutually exclusive group with --expand and --count.

Remove `-n, --nth` from the group so it can be used with these options.

src/cmd/flux-hostlist.py

@@ -65,7 +65,7 @@ def parse_args():
         action="store_true",
         help="Print the total number of hosts",
     )
-    group.add_argument(
+    parser.add_argument(
         "-n",
         "--nth",
         type=str,