Remove direct calls to public functions

[spalladino]

All txs will now start from a private entrypoint. Remove the ability to directly call a public function. This means removing the PublicTx type in a tx, as well as the flow in the PublicProcessor that uses the public kernel circuit with no previous kernel (remove that circuit as well).

Also, since txs will always have a private component, the tx hash will always be available as a first nullifier, so we can simplify the getTxHash method.