Only check history visibility when filtering

When filtering events to send to server we check more than just history
visibility. However when deciding whether to backfill or not we only
care about the history visibility.

synapse/handlers/federation.py

@@ -891,9 +891,11 @@ class FederationHandler(BaseHandler):
             get_prev_content=False,
         )
 
+        # We set `check_history_visibility_only` as we might otherwise get false
+        # positives from users having been erased.
         filtered_extremities = yield filter_events_for_server(
             self.store, self.server_name, list(extremities_events.values()),
-            redact=False,
+            redact=False, check_history_visibility_only=True,
         )
 
         if not filtered_extremities: