[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.

Release notes
-------------

.. toctree::
    :maxdepth: 1

    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14