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/gobject/performance-threaded.c

@@ -357,9 +357,6 @@ main (int   argc,
       return 0;
     }
 
-  if (n_threads)
-    g_thread_init (NULL);
-
   if (argc > 1)
     {
       for (i = 1; i < argc; i++)