Now that wgCheckUserEventTablesMigrationStage has been set to SCHEMA_COMPAT_NEW, the value can be assumed to always be SCHEMA_COMPAT_NEW and the configuration value can be removed.
This task will be done after T366781: Run maintenance script to delete entries only for use when reading old on WMF wikis. This means that this task can also remove references to cuc_only_for_read_old.
Acceptance criteria
- Remove wgCheckUserEventTablesMigrationStage and other migration related code from Hooks.php (assuming the value of SCHEMA_COMPAT_NEW where necessary)