Remove docs about Json\Expr now this feature is removed

docs/book/helpers/json.md

@@ -19,15 +19,3 @@ determine how to handle the content.
 ```php
 <?= $this->json($this->data) ?>
 ```
-
-> ### Enabling encoding using Laminas\Json\Expr
->
-> The JSON helper accepts an array of options that will be passed to `Laminas\Json\Json::encode()` and
-> used internally to encode data.
-> `Laminas\Json\Json::encode` allows the encoding of native JSON expressions using `Laminas\Json\Expr`
-> objects. This option is disabled by default. To enable this option, pass a boolean `true` to the
-> `enableJsonExprFinder` key of the options array:
->
-> ```php
-> <?= $this->json($this->data, ['enableJsonExprFinder' => true]) ?>
-> ```