vout: remove duplicate vout_Close() definition

It's a VLC_API defined in vlc_vout.h

src/video_output/vout_internal.h

@@ -232,15 +232,6 @@ void vout_Stop(vout_thread_t *);
  */
 void vout_StopDisplay(vout_thread_t *);
 
-/**
- * Destroys a vout.
- *
- * This function closes and releases a vout created by vout_Create().
- *
- * \param p_vout the vout to close
- */
-void vout_Close( vout_thread_t *p_vout );
-
 /**
  * Set the new source format for a started vout
  *