Push action summaries do not appear to be cleared when a user leaves a room

This issue has been migrated from [#14567](https://github.com/matrix-org/synapse/issues/14567).

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### Description

See: https://github.com/matrix-org/synapse/pull/14255/files/e79bc729b1ce9d5fad3c4109eac7dbeba03a9b2b#r1023137738

Discovered while working on that PR - it seems the summary table rows are left once a user leaves a room.

### Steps to reproduce

- join room, receive some notifications (but don't read it)
- leave room
- summary row remains in database

### Info

- synapse 1.72.0
- k8s/linux/workers/postgres