commit 448134d3cdbc54e5359ea33d387993b0defdaefa changed gdm to
import the session from systemd --user.

Unfortunately, it broke log in after log out. The problem is, certain
session specific environment variables from the previous session
were getting leaked into the new session.

This commit wipes the ones causing the most problems on the GDM side,
but gnome-session should also probably purge them from the systemd
--user environment when it exits.