config: Remove a repeated word from a logger warning

The warning for missing macaroon_secret_key was "missing missing".

synapse/config/key.py

@@ -56,7 +56,7 @@ class KeyConfig(Config):
         if not self.macaroon_secret_key:
             # Unfortunately, there are people out there that don't have this
             # set. Lets just be "nice" and derive one from their secret key.
-            logger.warn("Config is missing missing macaroon_secret_key")
+            logger.warn("Config is missing macaroon_secret_key")
             seed = bytes(self.signing_key[0])
             self.macaroon_secret_key = hashlib.sha256(seed).digest()