In MySQL 5.7, a follow-up to WL#6671 removed the unused
fields ha_innobase::lock and INNOBASE_SHARE::lock, but
MariaDB did not remove them, even though a counterpart of
WL#6671 itself was implemented as MDEV-7660 in
commit d665e79c.

INNOBASE_SHARE was removed in MDEV-16557. Thus, all that
needs to be removed is the unused member ha_innobase::lock
and related code.

Thanks to Monty (and Valgrind) for noticing that
ha_innobase::lock was uninitialized.