Remove link from admin-guide to schematron details as inappropriate

These are two different audiences. When deploying admin guide in isolation this produces a broken link.

If configuration of validation levels requires customization then the activity should move to the customization section.

docs/manual/docs/administrator-guide/managing-metadata-standards/configure-validation.md

@@ -1,6 +1,6 @@
 # Configuring validation levels {#configure-validation}
 
-Each standard defines validation levels (using schematron - see [Implementing schema plugins](../../customizing-application/implementing-a-schema-plugin.md)). By default, ISO19139 proposes validation using:
+Each standard defines validation levels. By default, ISO19139 proposes validation using:
 
 -   ISO rules
 -   INSPIRE rules (TG v1.3)