Get rid of unused EntityFieldValueGetting Session event

[SergeiPavlov]

Yet one hook with some overhead

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Note

Medium Risk
This is a breaking public API change for any consumer that subscribed to EntityFieldValueGetting; behavior for post-read EntityFieldValueGet is unchanged.

Overview
Removes the EntityFieldValueGetting session hook and the EntityFieldEventArgs type that fired before a field value was read. Field reads no longer raise a pre-get notification on Session.Events / system events; EntityFieldValueGet (after a successful read, with the value) is unchanged.

Entity.SystemBeforeGetValue no longer calls NotifyFieldValueGetting, which drops per-read event overhead on the hot path. SessionEventsTest was updated to stop subscribing to or asserting on the removed event.

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