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/child-test.c

@@ -164,9 +164,6 @@ main (int argc, char *argv[])
     }
 #endif
 
-#ifdef TEST_THREAD
-  g_thread_init (NULL);
-#endif
   main_loop = g_main_loop_new (NULL, FALSE);
 
 #ifdef G_OS_WIN32