Oracle introduced a Memcached plugin interface to the InnoDB
storage engine in MySQL 5.6. That interface is essentially a
fork of Memcached development snapshot 1.6.0-beta1 of an old
development branch 'engine-pu'.

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:

    innodb_api_bk_commit_interval
    innodb_api_disable_rowlock
    innodb_api_enable_binlog
    innodb_api_enable_mdl
    innodb_api_trx_level

Removing this code in one commit makes it possible to easily restore
it, in case it turns out to be needed later.