Once wgCheckUserEventTablesMigrationStage has been set to SCHEMA_COMPAT_NEW, the value can be assumed to always be SCHEMA_COMPAT_NEW and the configuration value removed.
Acceptance criteria
- Remove wgCheckUserEventTablesMigrationStage and update the code that uses it to assume that the value is SCHEMA_COMPAT_NEW
- Remove any code paths that are now unreachable by hard-coding the value