txprepare: revert 1fb9a078b6 (`psbt` field)

We're actually going to deprecate this, so don't add new features!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: ***REMOVE*** JSON-API: `txprepare` returns a psbt version of the created transaction

tests/test_wallet.py

@@ -307,7 +307,6 @@ def test_txprepare(node_factory, bitcoind, chainparams):
     wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 10)
 
     prep = l1.rpc.txprepare(outputs=[{addr: Millisatoshi(amount * 3 * 1000)}])
-    assert prep['psbt']
     decode = bitcoind.rpc.decoderawtransaction(prep['unsigned_tx'])
     assert decode['txid'] == prep['txid']
     # 4 inputs, 2 outputs (3 if we have a fee output).

wallet/walletrpc.c

@@ -490,7 +490,6 @@ static struct command_result *json_txprepare(struct command *cmd,
 	response = json_stream_success(cmd);
 	json_add_tx(response, "unsigned_tx", utx->tx);
 	json_add_txid(response, "txid", &utx->txid);
-	json_add_psbt(response, "psbt", utx->tx->psbt);
 	return command_success(cmd, response);
 }
 static const struct json_command txprepare_command = {