The XtraDB storage engine was already replaced by InnoDB
and disabled in MariaDB Server 10.2. Let us remove it altogether
to avoid dragging dead code around.

Replace some references to XtraDB with references to InnoDB.

rpl_get_position_info(): Remove.

Remove the mysql-test-run --suite=percona, because it only contains
tests specific to XtraDB, many of which were disabled already in
earlier MariaDB versions.