refactor(AztecNr): Clean up TransparentNote

[benesjan]

Remove unused functions and secret from TransparentNote as it's unused.

Note that there is an issue with potential nullifier collisions in case the same secretHash would be used 2 times. This can be tackled by implementing nonces as is described in this issue. Note that this will make it more tricky to add a note because we will need to somehow set the nonce when passing the note to pxe.addNote.

Either link the issue in the transparent note code or figure out a way how to handle it via some other mechanism.