Back out MSC2625 implementation (#7761)

changelog.d/7673.feature

changelog.d/7716.feature

changelog.d/7761.feature

synapse/handlers/sync.py

synapse/push/bulk_push_rule_evaluator.py

synapse/push/push_tools.py

synapse/rest/client/v1/push_rule.py

synapse/storage/data_stores/main/event_push_actions.py

...ores/main/schema/delta/58/07push_summary_unread_count.sql

tests/replication/slave/storage/test_events.py

tests/storage/test_event_push_actions.py