apply changes from review

changelog.d/6451.bugfix

-Change the index on the `e2e_cross_signing_signatures` table to be non-unique.
+Fix uploading multiple cross signing signatures for the same user.

synapse/storage/data_stores/main/schema/delta/56/signing_keys.sql

@@ -40,7 +40,8 @@ CREATE TABLE IF NOT EXISTS e2e_cross_signing_signatures (
     signature TEXT NOT NULL
 );
 
-CREATE INDEX e2e_cross_signing_signatures2_idx ON e2e_cross_signing_signatures(user_id, target_user_id, target_device_id);
+-- replaced by the index created in signing_keys_nonunique_signatures.sql
+-- CREATE UNIQUE INDEX e2e_cross_signing_signatures_idx ON e2e_cross_signing_signatures(user_id, target_user_id, target_device_id);
 
 -- stream of user signature updates
 CREATE TABLE IF NOT EXISTS user_signature_stream (