Fix wait_state (broken in 4.5.0)

[chatziko]

A finally clause was added to wait_state in 4.5.0 to remove the event from _async_events in case entity_state_changed is never called. However, this creashes in the normal case then entity_state_changed does get called, cause async_events.pop does not find the key and fails.

This tiny PR fixes this bug.

[acockburn]

Thanks!