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/onceinit.c

@@ -124,8 +124,6 @@ main (int   argc,
   g_assert (p == &dummy_value);
   p = initializer2();
   g_assert (p == &dummy_value);
-  /* setup threads */
-  g_thread_init (NULL);
   /* start multiple threads for initializer3() */
   g_mutex_lock (&tmutex);
   for (i = 0; i < N_THREADS; i++)