Debugger: Remove an unused function declaration

Change-Id: Ic936953eeeeeab82a73c11fbe80361326acff055
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggerengine.cpp

@@ -226,7 +226,6 @@ public:
                 this, &DebuggerEnginePrivate::resetLocation);
     }
 
-    void doRunEngine();
     void doShutdownEngine();
     void doShutdownInferior();