Remove redundant COALESCE()s around COUNT()s in database queries (#11570)

`COUNT()` never returns `NULL`. A `COUNT(*)` over 0 rows is 0 and a
`COUNT(NULL)` is also 0.

synapse/storage/databases/main/relations.py

@@ -390,7 +390,7 @@ class RelationsWorkerStore(SQLBaseStore):
             latest_event_id = row[0]
 
             sql = """
-                SELECT COALESCE(COUNT(event_id), 0)
+                SELECT COUNT(event_id)
                 FROM event_relations
                 INNER JOIN events USING (event_id)
                 WHERE