Remove dead handled_events set in invite_join (#9394)

This PR removes a set that was created and [initially used](https://github.com/matrix-org/synapse/commit/1d2a0040cff8d04cdc7d7d09d8f04a5d628fa9dd#diff-0bc92da3d703202f5b9be2d3f845e375f5b1a6bc6ba61705a8af9be1121f5e42R435-R436), but is no longer today.

May help cut down a bit on the time it takes to accept invites.