Android: remove duplicate declaration for m_contextMenuVisible

It's also declared in QtActivityDelegateBase.

Pick-to: 6.8
Change-Id: I5e526300f17ec5c808e99eee902c465906334713
Reviewed-by: Petri Virkkunen <petri.virkkunen@qt.io>

src/android/jar/src/org/qtproject/qt/android/QtActivityDelegate.java

@@ -329,12 +329,10 @@ class QtActivityDelegate extends QtActivityDelegateBase
     }
     // QtMenuInterface implementation end
 
-    private boolean m_contextMenuVisible = false;
-
     void onCreatePopupMenu(Menu menu)
     {
         QtNative.fillContextMenu(menu);
-        m_contextMenuVisible = true;
+        setContextMenuVisible(true);
     }
 
     @Override