fix: always use set for private identifiers

[paoloricciuti]

Closes #13965

Unless i'm missing something obvious there's no reason to not always transform PrivateIdentifiers that are also inside the class private state so no need to check for the left object to be this. JS will stop them to modify a private identifier which is not of the same class and if it is from the same class it's fine to use set to set it.

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