thread: reduce use of system_thread

The use of system_thread is now limited to joining.  We don't do that
for threads that we didn't create for ourselves, so we don't need to
call g_system_thread_self() to fill in system_thread for those.

glib/gthread.c

@@ -928,8 +928,6 @@ g_thread_self (void)
       thread->thread.func = NULL;
       thread->thread.data = NULL;
 
-      g_system_thread_self (&thread->system_thread);
-
       g_private_set (&g_thread_specific_private, thread);
     }