Disable calculating unread counts unless the config flag is enabled. (#13694)

This avoids doing work that will never be used (since the
resulting unread counts will never be sent in a /sync
response).

The negative of doing this is that unread counts will be
incorrect when the feature is initially enabled.

synapse/push/bulk_push_rule_evaluator.py

@@ -262,7 +262,12 @@ class BulkPushRuleEvaluator:
             # This can happen due to out of band memberships
             return
 
-        count_as_unread = _should_count_as_unread(event, context)
+        # Disable counting as unread unless the experimental configuration is
+        # enabled, as it can cause additional (unwanted) rows to be added to the
+        # event_push_actions table.
+        count_as_unread = False
+        if self.hs.config.experimental.msc2654_enabled:
+            count_as_unread = _should_count_as_unread(event, context)
 
         rules_by_user = await self._get_rules_for_event(event)
         actions_by_user: Dict[str, Collection[Union[Mapping, str]]] = {}