certdb: remove unused valid_months property

Property valid_months is no longer used anywhere in the code. It was
removed when the selfsign funcionality was dropped.

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>

ipaserver/install/certs.py

@@ -109,7 +109,6 @@ class CertDB(object):
             raise RuntimeError("Unable to determine the current directory: %s" % str(e))
 
         self.cacert_name = get_ca_nickname(self.realm)
-        self.valid_months = "120"
 
         # We are going to set the owner of all of the cert
         # files to the owner of the containing directory