[repository] Drop the entities.extid column and associated cache
This was not necessary anymore with promoted usage of the new data import API.
Turn repository's _type_extid_cache to _type_cache with only the entity's type
as key.
This introduces an backward incompatible change: entity_metas dict doesn't
contains anymore the extid key, but it doesn't seem used at all anywhere, so
this sounds acceptable.
Closes #15538317
.. _authplugin_module:
:mod:`cubicweb.pyramid.authplugin`
----------------------------------
.. automodule:: cubicweb.pyramid.authplugin
.. autoclass:: DirectAuthentifier
:show-inheritance:
:members: