Don't set the autorestart hint for gnome-session

If the autorestart hint is set, the process is forcefully killed and
restarted everytime it disconnects from XSMP, which break replacing the wm
and breaks alt-f2 r. If instead the hint is not set, the process
is monitored via SIGCHLD and only restarted when it dies by a signal.

https://bugzilla.gnome.org/show_bug.cgi?id=648384