Clang: Remove unused function

Change-Id: I78743489c6b5dc617c4a9966e83d3bf987707bc9
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangbackendcommunicator.h

@@ -127,7 +127,6 @@ private:
     void setupDummySender();
 
     void onConnectedToBackend();
-    void onDisconnectedFromBackend();
     void onEditorAboutToClose(Core::IEditor *editor);
 
     void logExecutableDoesNotExist();