Stop writing to event_reference_hashes (#12679)

This table is never read, since #11794. We stop writing to it; in future we can
drop it altogether.

synapse/storage/databases/main/events.py

@@ -36,7 +36,6 @@ from prometheus_client import Counter
 import synapse.metrics
 from synapse.api.constants import EventContentFields, EventTypes, RelationTypes
 from synapse.api.room_versions import RoomVersions
-from synapse.crypto.event_signing import compute_event_reference_hash
 from synapse.events import EventBase  # noqa: F401
 from synapse.events.snapshot import EventContext  # noqa: F401
 from synapse.storage._base import db_to_json, make_in_list_sql_clause
@@ -1600,11 +1599,6 @@ class PersistEventsStore:
             inhibit_local_membership_updates=inhibit_local_membership_updates,
         )
 
-        # Insert event_reference_hashes table.
-        self._store_event_reference_hashes_txn(
-            txn, [event for event, _ in events_and_contexts]
-        )
-
         # Prefill the event cache
         self._add_to_cache(txn, events_and_contexts)
 
@@ -1704,25 +1698,6 @@ class PersistEventsStore:
             values={"event_id": event_id, "expiry_ts": expiry_ts},
         )
 
-    def _store_event_reference_hashes_txn(self, txn, events):
-        """Store a hash for a PDU
-        Args:
-            txn (cursor):
-            events (list): list of Events.
-        """
-
-        vals = []
-        for event in events:
-            ref_alg, ref_hash_bytes = compute_event_reference_hash(event)
-            vals.append((event.event_id, ref_alg, memoryview(ref_hash_bytes)))
-
-        self.db_pool.simple_insert_many_txn(
-            txn,
-            table="event_reference_hashes",
-            keys=("event_id", "algorithm", "hash"),
-            values=vals,
-        )
-
     def _store_room_members_txn(
         self, txn, events, *, inhibit_local_membership_updates: bool = False
     ):