Remove a bunch of lingering g_thread_init()

After this patch, there is but one remaining use of g_thread_init(),
which is in tests/slice-threadinit.c, a testcase dedicated to testing
the functionality of gslice across a g_thread_init() boundary.

This testcase is pretty meaningless these days... probably we should
delete it.

tests/asyncqueue-test.c

@@ -185,8 +185,6 @@ main (int argc, char *argv[])
   gint   sort_interval;
   gchar *msg G_GNUC_UNUSED;
 
-  g_thread_init (NULL);
-
   basic_tests ();
 
   PRINT_MSG (("creating async queue..."));