Unnecessary contention on rw_lock_list_mutex in ibuf_dummy_index_create()

The function ibuf_dummy_index_create() is creating dummy dict_index_t objects for applying buffered changes. The rw_lock_create() call in this function should be totally unnecessary, because there should be no need to access dict_index_t::lock in those dummy objects at all. We should avoid registering the dummy objects in any global lists.

When MDEV-11634 implements a more compact change buffer format, that entire code should be removed, or only kept for the purpose of upgrading.