DEV: Remove legacy `MountWidget` usage in `docs-topic` component

Replaced conditional logic for `useGlimmerPostStream` with direct usage of the `Post` component, simplifying the `docs-topic` implementation.

assets/javascripts/discourse/components/docs-topic.gjs

@@ -72,13 +72,7 @@ export default class DocsTopic extends Component {
 
     <div class="topic-content">
       <h1>{{htmlSafe this.topic.fancyTitle}}</h1>
-
-      {{#if this.site.useGlimmerPostStream}}
       <Post @post={{this.model}} />
-      {{else}}
-        {{! template-lint-disable no-capital-arguments }}
-        <MountWidget @widget="post" @model={{this.model}} @args={{this.post}} />
-      {{/if}}
     </div>
 
     <a class="docs-nav-link more" href="/t/{{this.topic.id}}">