Drop more deprecated code
This follows up on changeset a8c1ea390400, in which code deprecated
using logilab.common.deprecation got dropped. Now we also drop code
deprecated using stdlib's warn(<msg>, DeprecationWarning).
Notice that, as a consequence of dropping old/new etypes aliases in
cubicweb/schema.py, we drop the import ETYPE_NAME_MAP (no longer
needed); but since other modules imported that name from
cubicweb.schema, we need to update the import statement to use
"cubicweb" directly.