Transaction is duplicated on the wire to signer

### Issue

Several hsmd protocol messages use `bitcoin_tx` which includes a `wally_tx` and a `wally_psbt`. However, `wally_psbt` already includes a `wally_tx`, resulting in duplication of the transaction on the wire.

We (VLS project) tried ignoring the top level `wally_tx`, but discovered that the two transactions differ in the amounts - possibly a different feerate is sometimes used.

In addition to not being DRY, the dupilcation results in additional memory usage in low-resource environments.