Simplify arguments to SignPSBTInput

Remove redundant arguments to SignPSBTInput -- since it needs several
bits of the PartiallySignedTransaction, pass in a reference instead of
doing it piecemeal. This saves us having to pass in both a PSBTInput and
its index, as well as having to pass in the CTransaction. Also avoid
redundantly passing the sighash_type, which is contained in the
PSBTInput already.

src/wallet/test/psbt_wallet_tests.cpp

@@ -60,7 +60,7 @@ BOOST_AUTO_TEST_CASE(psbt_updater_test)
     ssData >> psbtx;
 
     // Fill transaction with our data
-    FillPSBT(&m_wallet, psbtx, 1, false, true);
+    FillPSBT(&m_wallet, psbtx, SIGHASH_ALL, false, true);
 
     // Get the final tx
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);