macosx: remove one unneeded line in togglePlaylist:

(cherry picked from commit e375b238a70d235f21d3a378c85bd4b8500c90e7)

modules/gui/macosx/MainWindow.m

@@ -747,7 +747,6 @@ static VLCMainWindow *_o_sharedInstance = nil;
 
     if (b_dropzone_active && ([[NSApp currentEvent] modifierFlags] & NSAlternateKeyMask) != 0)
     {
-        b_dropzone_active = NO;
         [self hideDropZone];
         return;
     }