Get rid of unused OnGetFieldValue()

[SergeiPavlov]

Also:

• avoid boxing on every field reading in GetNormalizedFieldValue()

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Note

Medium Risk
Removes a public override point and touches the core field-read path; behavior change only for code that overrode OnGetFieldValue, with a low chance of subtle event/subscriber differences.

Overview
Removes the unused OnGetFieldValue override hook from Persistent (and the Entity / Structure calls that invoked it). Field-read side effects should rely on existing session EntityFieldValueGet events and entity event broker subscribers instead.

SystemGetValue / SystemGetValueCompleted and SessionEventAccessor’s matching notify methods are now generic over the read value type. Typed GetNormalizedFieldValue<T> passes the result through that pipeline directly, so value-type fields no longer get an extra object boxing step on every read.

Untyped GetNormalizedFieldValue / reference paths are unchanged in shape; session event args still expose object values when notifications run.

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