remove unused and unnecessary check for FederationDeniedError (#5645)

FederationDeniedError is a subclass of SynapseError, which is a subclass of
CodeMessageException, so if e is a FederationDeniedError, then this check for
FederationDeniedError will never be reached since it will be caught by the
check for CodeMessageException above.  The check for CodeMessageException does
almost the same thing as this check (since FederationDeniedError initialises
with code=403 and msg="Federation denied with %s."), so may as well just keep
allowing it to handle this case.

changelog.d/5645.misc

@@ -0,0 +1 @@
+Remove unused and unnecessary check for FederationDeniedError in _exception_to_failure.

synapse/handlers/e2e_keys.py

@@ -22,7 +22,7 @@
 
 from twisted.internet import defer
 
-from synapse.api.errors import CodeMessageException, FederationDeniedError, SynapseError
+from synapse.api.errors import CodeMessageException, SynapseError
 from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.types import UserID, get_domain_from_id
 from synapse.util.retryutils import NotRetryingDestination
@@ -350,9 +350,6 @@ def _exception_to_failure(e):
     if isinstance(e, NotRetryingDestination):
         return {"status": 503, "message": "Not ready for retry"}
 
-    if isinstance(e, FederationDeniedError):
-        return {"status": 403, "message": "Federation Denied"}
-
     # include ConnectionRefused and other errors
     #
     # Note that some Exceptions (notably twisted's ResponseFailed etc) don't