fix: remove oneof wrappers in v2

[bshaffer]

fixes #628

• Removes use of OneOf wrapper classes in V2 tests
• Removes use of OneOf wrapper classes in V2 samples
• Skips generating OneOf wrapper classes for MigrationMode::NEW_SURFACE_ONLY

[snippet-bot]

Here is the summary of changes.

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• tests/Unit/ProtoTests/BasicOneofNew/out/samples/BasicOneofNewClient/a_method.php:25, tag basic_generated_BasicOneofNew_AMethod_sync

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