[Wallet] Use RBF by default in QT only

GUI wallet uses RBF by default, regardless of -walletrbf.

RPC and debug console in the GUI remain unchanged; they don't
use RBF by default, unless launched with -walletrbf=1.

src/qt/walletmodel.cpp

@@ -737,8 +737,3 @@ int WalletModel::getDefaultConfirmTarget() const
 {
     return nTxConfirmTarget;
 }
-
-bool WalletModel::getDefaultWalletRbf() const
-{
-    return fWalletRbf;
-}