use standard Close button for consistency with other task dialogs

src/Gui/DlgDisplayProperties.ui

@@ -434,13 +434,6 @@
      </layout>
     </widget>
    </item>
-   <item row="3" column="0">
-    <widget class="QDialogButtonBox" name="buttonBox">
-     <property name="standardButtons">
-      <set>QDialogButtonBox::Close</set>
-     </property>
-    </widget>
-   </item>
   </layout>
  </widget>
  <layoutdefault spacing="6" margin="11"/>
@@ -531,21 +524,5 @@
     </hint>
    </hints>
   </connection>
-  <connection>
-   <sender>buttonBox</sender>
-   <signal>rejected()</signal>
-   <receiver>Gui::Dialog::DlgDisplayProperties</receiver>
-   <slot>reject()</slot>
-   <hints>
-    <hint type="sourcelabel">
-     <x>150</x>
-     <y>461</y>
-    </hint>
-    <hint type="destinationlabel">
-     <x>144</x>
-     <y>243</y>
-    </hint>
-   </hints>
-  </connection>
  </connections>
 </ui>

src/Gui/DlgDisplayPropertiesImp.cpp

@@ -192,11 +192,6 @@ void DlgDisplayPropertiesImp::setupConnections()
     connect(d->ui.buttonColorPlot, &ColorButton::clicked, this, &DlgDisplayPropertiesImp::onButtonColorPlotClicked);
 }
 
-void DlgDisplayPropertiesImp::showDefaultButtons(bool ok)
-{
-    d->ui.buttonBox->setVisible(ok);
-}
-
 void DlgDisplayPropertiesImp::changeEvent(QEvent *e)
 {
     if (e->type() == QEvent::LanguageChange) {
@@ -630,7 +625,6 @@ TaskDisplayProperties::TaskDisplayProperties()
 {
     this->setButtonPosition(TaskDisplayProperties::North);
     widget = new DlgDisplayPropertiesImp(false);
-    widget->showDefaultButtons(false);
     taskbox = new Gui::TaskView::TaskBox(QPixmap(), widget->windowTitle(),true, nullptr);
     taskbox->groupLayout()->addWidget(widget);
     Content.push_back(taskbox);