It does not make sense anymore to update the config of a source instance
(subclass of cubicweb.server.sources.AbstractSource) now that they are always
built from database information (CWSource). In datafeed and ldapfeed, we
move all code from "update_config" method in "init" method.

This changeset fixes LDAPFeedUserDeletionTC.test_a_filter_inactivate() failure
(unittest_ldapsource.py) introduces in previous changeset.