[server] Drop update_config method of source 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.