MDEV-17614 flags INSERT…ON DUPLICATE KEY UPDATE unsafe for statement-based
replication when there are multiple unique indexes. This correctly fixes
something whose attempted fix in MySQL 5.7
in mysql/mysql-server@c93b0d9a972cb6f98fd445f2b69d924350f9128a
caused lock conflicts. That change was reverted in MySQL 5.7.26
in mysql/mysql-server@066b6fdd433aa6673622341f1a2f0a3a20018043
(with a substantial amount of other changes).

In MDEV-17073 we already disabled the unfortunate MySQL change when
statement-based replication was not being used. Now, thanks to MDEV-17614,
we can actually remove the change altogether.

This reverts commit 8a346f31 (MDEV-17073)
and mysql/mysql-server@c93b0d9a972cb6f98fd445f2b69d924350f9128a while
keeping the test cases.