Remove storage slot from note header #11857

We want to remove this from the user-facing representation of notes, and so extracting the storage slot out of the header is a good place to start. The slot is only needed when creating, reading or nullifying, which is always managed by the state variable, which will be the one providing the storage slot. Manual usage of the slot may lead to breakage of state variable invariants.

Removing the storage slot will likely require changing the compute_note_hash and compute_nullifier_without_context (temporary until #11858) functions to not just take self but also storage_slot. We already special-case the storage slot in the autogeneration of the hash functions so this should not be very complicated.