Drop the unused _relogin helper

Nothing called it — not the adapter, not the tests, not the gateway. It
would not have forced a re-login either: it cleared self._token, but
_ensure_token() checks the configured token and the existing client, so
a configured token made it return True without logging in again. Token
expiry is handled by the reconnect path.

adapter.py

@@ -319,11 +319,6 @@ class ChattoAdapter(BasePlatformAdapter):
         client = await self._get_chatto_client()
         return client is not None
 
-    async def _relogin(self) -> bool:
-        """Force re-login (token expired)."""
-        self._token = None
-        return await self._ensure_token()
-
 
     # ------------------------------------------------------------------ #
     # Connection