Remove functionality to migrate from MyISAM, and remove tests of whether
the existing database is using this.
Support for MyISAM was dropped in Moodle 2.9 (MDL-46064) so it should
not be possible for any subsequent Moodle version to be using this.
The only remaining MyISAM-aware code is the environment check via
check_database_storage_engine() to prevent installation/upgrade if the
database is set to use this.
Ilya Tregubov