Oracle introduced a Memcached plugin interface to the InnoDB storage engine in MySQL 5.6. That interface is essentially a fork of what seems like an abandoned development branch of Memcached. To my knowledge, there have not been any updates to the Memcached code between MySQL 5.6 and 5.7; only bug fixes and extensions related to the Oracle modifications.
The Memcached plugin is not part of the MariaDB Server. Therefore it does not make sense to include the InnoDB interfaces for the Memcached plugin, or to have any related configuration parameters:
While there is a request to implement Memcached in MariaDB (MDEV-4674), it cannot possibly be completed before the MariaDB Server 10.2 reaches Generally Available status. Removing this code in one commit makes it possible to easily restore it later, in case it turns out to be needed.