Get rid of unused EntityFieldValueGetCompleted event

[SergeiPavlov]

Note

Medium Risk
This is a breaking public API change for any subscriber to Session.Events.EntityFieldValueGetCompleted; behavior on failed field reads no longer surfaces through that event.

Overview
Removes the EntityFieldValueGetCompleted session event and its EntityFieldValueGetCompletedEventArgs type, along with the internal SystemGetValueCompleted hook on Persistent, Entity, and Structure.

Field read paths in Persistent (GetNormalizedFieldValue, GetReferenceKey) no longer wrap reads in try/catch solely to raise a completion event with success or failure. EntityFieldValueGet and the existing get-time hooks are unchanged; only the post-read “completed” notification (including on exceptions) is gone.

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