Stop writing to the event_txn_id table

synapse/storage/schema/__init__.py

@@ -114,19 +114,15 @@
 Changes in SCHEMA_VERSION = 80
     - The event_txn_id_device_id is always written to for new events.
     - Add tables for the task scheduler.
+
+Changes in SCHEMA_VERSION = 81
+    - The event_txn_id is no longer written to for new events.
 """
 
 
 SCHEMA_COMPAT_VERSION = (
-    # Queries against `event_stream_ordering` columns in membership tables must
-    # be disambiguated.
-    #
-    # The threads_id column must written to with non-null values for the
-    # event_push_actions, event_push_actions_staging, and event_push_summary tables.
-    #
-    # insertions to the column `full_user_id` of tables profiles and user_filters can no
-    # longer be null
-    76
+    # The `event_txn_id_device_id` must be written to for new events.
+    80
 )
 """Limit on how far the synapse codebase can be rolled back without breaking db compat