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/mainloop-test.c

@@ -398,14 +398,12 @@ recurser_start (gpointer data)
   return TRUE;
 }
 
-int 
+int
 main (int   argc,
       char *argv[])
 {
   gint i;
 
-  g_thread_init (NULL);
-
   context_array = g_ptr_array_new ();
 
   crawler_array = g_ptr_array_new ();