Remove option_payload_ptr; redundant to offset_of

The `option_payload_ptr` intrinsic is no longer required as `offset_of` supports traversing enums (#114208). This PR removes it in order to dogfood offset_of (as suggested at #106655 (comment)). However, it will not build until those changes reach beta (which I think is within the next 8 days?) so I've opened it as a draft.