VstEffect: removed unused slot declaration

The openPlugin() slot in VstEffectControls class is not implemented,
therefore do not declare it as otherwise MOC references it.

plugins/vst_effect/VstEffectControls.h

@@ -67,7 +67,6 @@ public:
 
 protected slots:
 	void updateMenu( void );
-	void openPlugin( void );
 	void managePlugin( void );
 	void openPreset( void );
 	void savePreset( void );