Don't pull out state in compute_event_context for unconflicted state

[erikjohnston]

When we compute the context of an event with only one prev event we pull out the full state from the DB:

synapse/synapse/state/__init__.py

Lines 373 to 377 in e5716b6

	if len(state_group_ids_set) == 1:
	(state_group_id,) = state_group_ids_set
	state = await self._state_storage_controller.get_state_for_groups(
	state_group_ids_set
	)

This often then goes unused, so we should stop doing that.