python: Remove redundant dest for argparse arguments.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/management/commands/send_custom_email.py

@@ -19,7 +19,6 @@ class Command(ZulipBaseCommand):
         parser.add_argument('--entire-server', action="store_true", default=False,
                             help="Send to every user on the server.")
         parser.add_argument('--markdown-template-path', '--path',
-                            dest='markdown_template_path',
                             required=True,
                             type=str,
                             help='Path to a Markdown-format body for the email.')