clang-tidy, qt: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/qt/sendcoinsdialog.cpp

@@ -64,11 +64,7 @@ int getIndexForConfTarget(int target) {
 SendCoinsDialog::SendCoinsDialog(const PlatformStyle *_platformStyle, QWidget *parent) :
     QDialog(parent, GUIUtil::dialog_flags),
     ui(new Ui::SendCoinsDialog),
-    clientModel(nullptr),
-    model(nullptr),
     m_coin_control(new CCoinControl),
-    fNewRecipientAllowed(true),
-    fFeeMinimized(true),
     platformStyle(_platformStyle)
 {
     ui->setupUi(this);