Fix performance of `check_state_groups_and_bump_deletion` (#18141)

Regressed as part of https://github.com/element-hq/synapse/pull/18107

This does two things:
1. Only check if the state groups have been deleted when calculating the
event context (as that's when we will insert them). This avoids lots of
checks for read operations.
2. Don't lock the `state_groups` rows when doing the check. This adds
overhead, and it doesn't prevent any races.