[gail] Clean the code to check the redundan object on the show watcher

[gail] Clean the code to check the redundan object on the show watcher

The watcher doesn't add a window if is a redundant object. This
patch fixes two things:

 * The check was made twice.
 * It uses a check with the string "redundant object", when the
   defined role ATK_ROLE_REDUNDANT_OBJECT is available