remove custom drill through from users guide

docs/users-guide/start.md

@@ -17,6 +17,5 @@
 *   [Creating SQL Templates](13-sql-parameters.md)
 *   [Viewing X-ray reports](14-x-rays.md)
 *   [Getting alerts](15-alerts.md)
-*   [Customizing drill-through](16-customizing.drill-through.md)
 
 Let's get started with an overview of [What Metabase does](01-what-is-metabase.md).