[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.

[system]
db-driver   = sqlite
db-name     = tmpdb-extern
db-encoding = UTF-8