Remove note hash's nullifier counter.

[LeilaWang]

Problem

Currently we assign a nullifier counter to a pending note hash. If this value is not zero, it means the note hash is nullified in the same tx by the nullifier with that specific counter.

We use this counter to check that, when reading a pending note, the read request is emitted before the note hash's nullified.

Although this check makes sense, it doesn't align with how reading a settled note hash works. A read request for a settled note hash is valid as long as the membership check succeeds, even if its nullifier has been emitted.

Solution

Removing this check because this shouldn't be enforced by the protocol only for reading pending note hashes. The developers should be able to write the same code and expect the same outcome for reading either pending or settled note hashes.

[LeilaWang]

Closed by #7294