data: ensure systemd environment is sanitized when shell exits

data: ensure systemd environment is sanitized when shell exits

When mutter is acting as a display server it sets a number of
environment variables in the user's session. These variables
tell applications where the display server's sockets are.

When the shell exits at logout time it leaves these environment
variables in the systemd --user environment, which can confuse
subsequent sessions.

This commit clears up the environment on exit.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1129