Get rid of AdjustFieldValue()

[SergeiPavlov]

This virtual method has never been overridden

And it adds some overhead to every Entity field access.

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Note

Low Risk
Removes an unused public virtual API; behavior is unchanged unless external subclasses overrode AdjustFieldValue, which the PR states did not happen in this repo.

Overview
Removes the AdjustFieldValue extension points from Persistent (read and write overloads) and stops calling them on every field get/set.

Field reads now pass the accessor value straight into SystemGetValue / SystemGetValueCompleted without an extra object round-trip and cast. Writes assign the incoming value directly via SetUntypedValue instead of transforming it through AdjustFieldValue first.

This is a hot-path cleanup: the hooks were never overridden in the codebase but still added work on each entity field access.

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[snaumenko-st]

Good catch!