Remove deprecated support for expire time

In #2936736: Expired items never updated the performance of populating the queue of expired items has been greatly improved by no longer performing an Entity::load() on every single item. This means we can no longer retrieve the expire time from the fields, so this is no longer populated on the expired items that are being queued, and RefreshExpiredFieldEventInterface::getExpireTime() has been deprecated.

This is a backwards compatibility break, but this is considered to be an acceptable tradeoff for the increased performance and more reliable processing of the queue. Thanks to this change all expired items are being processed and the expire time is no longer important.

Developers that require the expire time for some reason can simply retrieve this by loading the entity themselves.