daemon: Stop exposing a GNOME_KEYRING_PID variable

We exit with the DBus session bus. Remove this clutter from the
environment.  PAM module no longer cares about the lifetime of
the deamon, except in one case: where it started the daemon in
order to change a password and the auto_start argument wasn't set.

https://bugzilla.gnome.org/show_bug.cgi?id=725801