refactor: remove <util/system.h> include from wallet.h

Since we no longer store a ref to the global `ArgsManager`
inside the wallet, we can move the util/system.h
include to the cpp.

This dependency removal opened a can of worms, as few
other places were, invalidly, depending on the wallet's
header including it.

src/qt/sendcoinsdialog.cpp

@@ -203,7 +203,7 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         if (model->wallet().hasExternalSigner()) {
             //: "device" usually means a hardware wallet.
             ui->sendButton->setText(tr("Sign on device"));
-            if (gArgs.GetArg("-signer", "") != "") {
+            if (model->getOptionsModel()->hasSigner()) {
                 ui->sendButton->setEnabled(true);
                 ui->sendButton->setToolTip(tr("Connect your hardware wallet first."));
             } else {