Old dead users cause large number of inserts to event_push_actions

[matrixbot]

This issue has been migrated from #5569.

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matrix.org inserts a lot of rows into event_push_actions (and _staging) for users that have long since disappeared. We should do something to reduce that.

(We insert a row per user per room for each event that notifies people, so messages in large rooms cause a lot of rows to be inserted)