[ldapfeed] use LDAP_SCOPES to find user-scope
We already have LDAP_SCOPES dict defining possible scopes, this avoid possible code injection from config...
Thus we can drop global variables already defined in LDAP_SCOPES since they are not used directly anymore.
raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")