MDEV-9026: Revert patch for MDEV-6069
MDEV-9026: Revert patch for MDEV-6069

Post-fix:
Reverting the patch for MDEV-6069 brought some ALTERs with
ENGINE=MYISAM back into the mysql_system_tables_fix.sql
script. As a result, running mysql_upgrade with global
enforce_storage_engine=INNODB (or any other non-MyISAM
engine, for that matter) would fail.

Fixed by locally unsetting enforce_storage_engine in the
upgrade script.

davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,ollie314/server,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2