main: Remove NOTIFY_SOCKET from environment

It is only used exactly once to notify systemd about the service
startup. So unset it as soon as possible as it can leak into
subprocesses we spawn otherwise.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/741

src/main.c

@@ -532,7 +532,9 @@ main (int argc, char **argv)
   shell_init_debug (g_getenv ("SHELL_DEBUG"));
 
   shell_dbus_init (meta_get_replace_current_wm ());
-  sd_notify (0, "READY=1");
+  /* We only use NOTIFY_SOCKET exactly once; unset it so it doesn't remain in
+   * our environment. */
+  sd_notify (1, "READY=1");
   shell_a11y_init ();
   shell_perf_log_init ();
   shell_introspection_init ();