qml: fix context menu not opening in playlist delegate

Regression since d34c0872,
and `this` here is not necessary. It is not clear why it has
been used before.

modules/gui/qt/playlist/qml/PlaylistDelegate.qml

@@ -254,7 +254,7 @@ T.ItemDelegate {
             }
 
             if (contextMenu && mouse.button === Qt.RightButton)
-                contextMenu.popup(index, this.mapToGlobal(mouse.x, mouse.y))
+                contextMenu.popup(index, mapToGlobal(mouse.x, mouse.y))
         }
 
         onDoubleClicked: (mouse) => {