Dont show service certificates for removed modules

There is a risk of no-matching CN if the module is reinstalled later. However in the anterior approach (dont remove certificates of removed modules) the risk was the same if the module dependences were purged.

Since the problem with CN can happens with the two approaches, I like more to remove the certificates which is clearer for the user.  (see http://trac.zentyal.org/ticket/6014 )