Consider only inserting events we actually want to purge into `events_to_purge`

This issue has been migrated from [#13419](https://github.com/matrix-org/synapse/issues/13419).

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In [`purge_history`](https://github.com/matrix-org/synapse/blob/2ee0b6ef4b78bada535beb30301cf0e01cbb7d81/synapse/storage/databases/main/purge_events.py#L30) we create a temporary table (`events_to_purge`, which we drop at the end of the purge) to store events we want to purge, alongside with whether or not we should delete them. It feels like there's room for improvement there, as in theory we should be able to only insert events we want to purge. I'm not sure how complicated this would be in practice, though. According to the code, we seem to use all the events in `events_to_purge`, even those not slated for deletion, to figure out state groups to remove. However according to comments it also seems to think that all events in that table _are_ slated for deletion.