Mostly drop old pre-actor user schemas
This removes most of the pre-actor user and user_text columns, and the
$wgActorTableSchemaMigrationStage setting that used to determine
whether the columns were used.
rev_user and rev_user_text remain in the code, as on Wikimedia wikis the
revision table is too large to alter at this time. A future change will
combine that with the removal of rev_comment, rev_content_model, and
rev_content_format (and the addition of rev_comment_id and rev_actor).
ActorMigration's constructor continues to take a $stage parameter, and
continues to have the logic for handling it, for the benefit of
extensions that might need their own migration process. Code using
ActorMigration for accessing the core fields should be updated to use
the new actor fields directly. That will be done for in a followup.