OSX interface: remove dummy lock acquisition

It is only used from a single thread

modules/gui/macosx/intf.m

@@ -1460,7 +1460,6 @@ static void manage_cleanup( void * args )
     while( true )
     {
         NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
-        vlc_mutex_lock( &p_intf->change_lock );
 
         if( !p_input )
         {
@@ -1490,8 +1489,6 @@ static void manage_cleanup( void * args )
         /* Manage volume status */
         [self manageVolumeSlider];
 
-        vlc_mutex_unlock( &p_intf->change_lock );
-
         msleep( INTF_IDLE_SLEEP );
 
         [pool release];