[hooks/integrity] kill CheckUniqueHook
It's the backend's job.
The noticeable consequence (as can be seen in tests updated here) is that
error message are a bit less user-friendly.
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py