[mail] drop hijack_user usage
The transaction is now commited before notification. We can create a new session
directly and drop `hijack_user` usage.
This prepare #2781782
#! /bin/sh -ecase "$1" in remove) if [ -x "/etc/init.d/cubicweb-ctl" ]; then invoke-rc.d cubicweb-ctl stop || true fi ;;esac#DEBHELPER#exit 0