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.

Github-Pull: #14588
Rebased-From: 0f5bda2bd9

src/rpc/rawtransaction.cpp

@@ -1312,7 +1312,6 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
     //   signature, but have not combined them yet (e.g. because the combiner that created this
     //   PartiallySignedTransaction did not understand them), this will combine them into a final
     //   script.
-
     CMutableTransaction mtx;
     bool complete = FinalizeAndExtractPSBT(psbtx, mtx);
 

src/script/sign.cpp

@@ -249,6 +249,9 @@ namespace {
     return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
 }
 
+    PSBTInput& input = psbt.inputs.at(index);
+    const CMutableTransaction& tx = *psbt.tx;
+
 class SignatureExtractorChecker final : public BaseSignatureChecker
 {
 private: