[docs] Remove rule_overridable in quizaccess page

This feature was never implemented in this way, the docs are incorrect.

Author: marxjohnson

docs/apis/plugintypes/quizaccess/_examples/rule_overridable.md

@@ -66,21 +66,3 @@ import RuleDescription from './_examples/rule.md';
     example={RuleFile}
     description={RuleDescription}
 />
-
-import RuleOverridableFile from '!!raw-loader!./_examples/rule_overridable.php';
-import RuleOverridableDescription from './_examples/rule_overridable.md';
-
-<ComponentFileSummary
-    filepath="/rule.php"
-    summary="Rule definition class with override"
-    plugintype="quizaccessrule"
-    pluginname="pluginname"
-    example={RuleOverridableFile}
-    description={RuleOverridableDescription}
-/>
-
-:::info
-
-Implementing `rule_overridable` is not required but can enhance the usability of the rule.
-
-:::