[sobjects] Drop custom unique constraint checks in user registration service
This constraints (on unique CWUser's login and EmailAddress's login) are
handled by the backend.
This follows "[hooks/integrity] kill CheckUniqueHook".
Extend test for the ValidationError along the way.
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py