Combine STTx::checkSign overloads:

Callers don't need to specify the signing key -- they're just retrieving
the key from the SerializedTransaction and then passing it back.

This simplifies Ed25519 implementation.

src/ripple/app/transactors/Transactor.cpp

@@ -251,7 +251,7 @@ TER Transactor::preCheck ()
     if (!mTxn.isKnownGood ())
     {
         if (mTxn.isKnownBad () ||
-            (!(mParams & tapNO_CHECK_SIGN) && !mTxn.checkSign (mSigningPubKey)))
+            (!(mParams & tapNO_CHECK_SIGN) && !mTxn.checkSign()))
         {
             mTxn.setBad ();
             m_journal.warning << "apply: Invalid transaction (bad signature)";

src/ripple/app/tx/Transaction.cpp

@@ -69,7 +69,7 @@ Transaction::pointer Transaction::sharedTransaction (
 bool Transaction::checkSign () const
 {
     if (mFromPubKey.isValid ())
-        return mTransaction->checkSign (mFromPubKey);
+        return mTransaction->checkSign();
 
     WriteLog (lsWARNING, Ledger) << "Transaction has bad source public key";
     return false;