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/slice-test.c

@@ -264,8 +264,6 @@ main (int   argc,
       seedp = &seed32;
     }
 
-  g_thread_init (NULL);
-
   if (argc <= 1)
     usage();