Revert "gmain: Add private API to create Unix child watch that uses waitid()"

This reverts commit 93bf37ce15.

glib/glib-private.c

@@ -41,8 +41,7 @@ glib__private__ (void)
     g_get_worker_context,
 
     g_check_setuid,
-    g_main_context_new_with_next_id,
-    g_child_watch_source_new_nowait
+    g_main_context_new_with_next_id
   };
 
   return &table;