Remove old qt code

new round of removal following the report of @benj5378  .

.

src/Gui/Dialogs/DlgSettingsColorGradientImp.cpp

@@ -88,13 +88,10 @@ void DlgSettingsColorGradientImp::setupConnections()
     group->setExclusive(true);
     group->addButton(ui->radioButtonFlow);
     group->addButton(ui->radioButtonZero);
-#if QT_VERSION >= QT_VERSION_CHECK(5,15,0)
+
     connect(group, &QButtonGroup::idClicked,
             this, &DlgSettingsColorGradientImp::colorModelChanged);
-#else
-    connect(group, qOverload<int>(&QButtonGroup::buttonClicked),
-            this, &DlgSettingsColorGradientImp::colorModelChanged);
-#endif
+
     connect(ui->comboBoxModel, qOverload<int>(&QComboBox::currentIndexChanged),
             this, &DlgSettingsColorGradientImp::colorModelChanged);