Remove header color override entirely

This is unnecessary now that ToolBar colors come from the QQC2 style in
the manner that we need here.

discover/qml/UpdatesPage.qml

@@ -113,8 +113,6 @@ DiscoverPage
     readonly property int unselected: (updateModel.totalUpdatesCount - updateModel.toUpdateCount)
 
     header: ToolBar {
-        Kirigami.Theme.colorSet: Kirigami.Theme.Header
-        Kirigami.Theme.inherit: false
         visible: (updateModel.totalUpdatesCount > 0 && resourcesUpdatesModel.isProgressing) || updateModel.hasUpdates
 
         CheckBox {