applets/taskmanager: Use `tasksModel.activeTask` in GroupDialog

We don't really use the binding, and simply use tasksModel.activeTask is
enough.

applets/taskmanager/package/contents/ui/GroupDialog.qml

@@ -33,14 +33,12 @@ PlasmaCore.Dialog {
     property alias overflowing: scrollView.overflowing
     property var _oldAppletStatus: PlasmaCore.Types.UnknownStatus
 
-    property var activeTask: null
-
     function findActiveTaskIndex() {
-        if (!activeTask) {
+        if (!tasksModel.activeTask) {
             return;
         }
         for (let i = 0; i < groupListView.count; i++) {
-            if (tasksModel.makeModelIndex(visualParent.itemIndex, i) === activeTask) {
+            if (tasksModel.makeModelIndex(visualParent.itemIndex, i) === tasksModel.activeTask) {
                 groupListView.positionViewAtIndex(i, ListView.Contain); // Prevent visual glitches
                 groupListView.currentIndex = i;
                 return;

applets/taskmanager/package/contents/ui/main.qml

@@ -184,19 +184,6 @@ MouseArea {
         }
     }
 
-    Connections {
-        target: tasksModel
-
-        function onActiveTaskChanged() {
-            if (!plasmoid.configuration.groupPopups) {
-                return;
-            }
-            if (tasksModel.activeTask.parent.valid) {
-                groupDialog.activeTask = tasksModel.activeTask;
-            }
-        }
-    }
-
     TaskManager.VirtualDesktopInfo {
         id: virtualDesktopInfo
     }