Delete aborted QDrag

Aborted QDrag object has never been removed, moreover there was no way
to know a previous drag-and-drop operation status

Task-number: QTBUG-53990
Change-Id: I2b6a7aad86d94b9d5083f9799fe54752c251660b
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/plugins/platforms/xcb/qxcbdrag.cpp

@@ -1121,6 +1121,9 @@ void QXcbDrag::cancel()
     QBasicDrag::cancel();
     if (current_target)
         send_leave();
+
+    // remove canceled object
+    currentDrag()->deleteLater();
 }
 
 // find an ancestor with XdndAware on it