Once enough time has passed since T361206: Stop writing old for user agent schema migration on WMF wikis, the agent column (e.g. cuc_agent, cupe_agent) should be removed from the CheckUser result tables (e.g. cu_changes, cu_log_event).
Acceptance criteria
- The $wgCheckUserUserAgentTableMigrationStage config is dropped, replacing all uses with an assumption of read and write new
- The cuc_agent, cule_agent, and cupe_agent columns are removed from the cu_changes, cu_log_event and cu_private_event tables respectively in the master branch of CheckUser
- A Schema-change-in-production ticket is filed to track applying the schema change to WMF wikis - T418465