[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.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)