keep-alive: Remove custom hash table handling

The gjs_g_hash_table_steal_one() function was probably written before
GHashTableIter was in the API. We can replace it by GHashTableIter and
remove the custom API.

For some reason, the custom API was causing the keep-alive finalizer to
crash under SpiderMonkey 38.

https://bugzilla.gnome.org/show_bug.cgi?id=776966