MSC4140: don't cancel delayed state on own state (#17810)

When a user sends a state event, do not cancel their own delayed events
for the same piece of state.

For context, see [the relevant section in the
MSC](https://github.com/matrix-org/matrix-spec-proposals/blob/a09a883d9a013ac4b6ffddebd7ea87a827d211b9/proposals/4140-delayed-events-futures.md#delayed-state-events-are-cancelled-by-a-more-recent-state-event).