Fix duplicate logging of exceptions in transaction processing (#9780)

There's no point logging this twice.

changelog.d/9780.bugfix

@@ -0,0 +1 @@
+Fix duplicate logging of exceptions thrown during federation transaction processing.

synapse/federation/transport/server.py

@@ -425,13 +425,9 @@ class FederationSendServlet(BaseFederationServlet):
             logger.exception(e)
             return 400, {"error": "Invalid transaction"}
 
-        try:
         code, response = await self.handler.on_incoming_transaction(
             origin, transaction_data
         )
-        except Exception:
-            logger.exception("on_incoming_transaction failed")
-            raise
 
         return code, response