Remove the message attribute from exceptions

This is causing python2 tests print ugly warnings about the
deprecation of the `message` attribute in python2.6.

https://pagure.io/freeipa/issue/7131

Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipalib/messages.py

@@ -129,11 +129,6 @@ class PublicMessage(UserWarning):
             data=self.kw,
         )
 
-    if six.PY3:
-        @property
-        def message(self):
-            return str(self)
-
 
 class VersionMissing(PublicMessage):
     """