fix(coderd): remove deprecated AITaskSidebarApp column (#20680)

This column was no longer used in `v2.28` and the codersdk field
deprecated. Both can now be dropped in `v2.29`.

Closes coder/internal#974

coderd/database/queries/workspacebuilds.sql

@@ -240,7 +240,6 @@ UPDATE
 	workspace_builds
 SET
 	has_ai_task = @has_ai_task,
-	ai_task_sidebar_app_id = @sidebar_app_id,
 	has_external_agent = @has_external_agent,
 	updated_at = @updated_at::timestamptz
 WHERE id = @id::uuid;