Fix config migration option

Make sure that --enable-migration in config plugin works correctly
and user can use to both enable and disable migration.

https://fedorahosted.org/freeipa/ticket/2150

ipalib/plugins/config.py

@@ -184,11 +184,6 @@ class config_mod(LDAPUpdate):
     __doc__ = _('Modify configuration options.')
 
     def pre_callback(self, ldap, dn, entry_attrs, attrs_list, *keys, **options):
-        if 'ipamigrationenabled' in entry_attrs:
-            if entry_attrs['ipamigrationenabled']:
-                entry_attrs['ipamigrationenabled'] = 'TRUE'
-            else:
-                entry_attrs['ipamigrationenabled'] = 'FALSE'
         if 'ipadefaultprimarygroup' in entry_attrs:
             group=entry_attrs['ipadefaultprimarygroup']
             try: