DEV: Remove redundant lines of code.

Engines are enabled by default so there is no need to specify Discourse
specific engines in the object.

app/assets/javascripts/pretty-text/addon/pretty-text.js

@@ -41,15 +41,11 @@ export function buildOptions(state) {
     watchedWordsLink,
   } = state;
 
+  // These are markdown-it rules defined in https://github.com/markdown-it/markdown-it#manage-rules.
   let features = {
     "bold-italics": true,
     "auto-link": true,
-    mentions: true,
-    bbcode: true,
-    quote: true,
     html: true,
-    "category-hashtag": true,
-    onebox: true,
     linkify: linkify !== false,
     newline: !siteSettings.traditional_markdown_linebreaks,
   };