Merge commit 2b6f8044 introduced
the debug injection point dict_sys_mutex_avoid to make the test
innodb.instant_alter_crash work even after the MDEV-23991 fix
(commit afc9d00c).

Thanks to DDL being atomic and crash-safe in MariaDB 10.6
(mainly thanks to commit 7762ee5d)
we do not actually need this hack anymore.