[connection] remove ensure_cnx_set context manager uses
It has been deprecated in a previous changeset and is now a no-op.
Related to #2919309.
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0