[repoapi,session] remove all session-as-cnx backward compat
The `dbapi` being gone, we now can drop the session object
bw-compatibility layer. This will allow further simplifications, such
as folding ClientConnection and Connection (without too much pain),
and then having persistent sessions.
Related to #3933480.
usr/lib/PY_VERSION/*-packages/cubicweb/web
usr/lib/PY_VERSION/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc