[markdown] Remove client-side syntax highlighting (#189)

* Remove client side highlighting

* Update CSS rules to render plain code blocks with expected left border

* Add back codeColor background color config

* Ensure all pre code blocks use Highlight.js classes

lib/core/Site.js

@@ -128,12 +128,7 @@ class Site extends React.Component {
                 }}
               />
             ))}
-          <script src={`//cdnjs.cloudflare.com/ajax/libs/highlight.js/${highlightVersion}/highlight.min.js`}></script>
-          <script
-            dangerouslySetInnerHTML={{
-              __html: `hljs.initHighlightingOnLoad();`
-            }}
-          />
+
         </body>
       </html>
     );