fix: remove stale .client attribute access in send_message command

APRSDClient no longer has a .client property after the driver refactor
(commit 1c39546). Instantiating APRSDClient() is sufficient to trigger
connection via auto_connect=True.

aprsd/cmds/send_message.py

@@ -130,7 +130,7 @@ def send_message(
                 sys.exit(0)
 
     try:
-        APRSDClient().client  # noqa: B018
+        APRSDClient()  # noqa: B018
     except LoginError:
         sys.exit(-1)