Remove unused `events_by_room` (#10421)

It looks like it was first used and introduced in https://github.com/matrix-org/synapse/commit/5130d80d79fe1f95ce03b8f1cfd4fbf0a32f5ac8#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8R305 but the 

But the usage was removed in https://github.com/matrix-org/synapse/commit/4c6a31cd6efa25be4c9f1b357e8f92065fac63eb#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8