website: remove references to the deleted `disc.html` page (#4119)

Refs: https://github.com/transloadit/uppy/pull/3363

website/src/robots.txt

@@ -1,2 +1 @@
 User-agent: *
-Disallow: /disc.html

website/themes/uppy/layout/partials/plugin_list.ejs

@@ -33,12 +33,6 @@
   </table>
 </div>
 
-<iframe scrolling="no" seamless="seamless" class="Disc" src="/disc.html"></iframe>
-<p>
-  This graph is built with
-  <a href="https://twitter.com/hughskennedy">Hugh Kennedy</a>’s excellent <a href="https://hughsk.io/disc/">disc</a>.
-</p>
-
 <h2 id="browser-support">Browser Support</h2>
 
 <p>We aim to support recent versions of Chrome, Firefox, Safari and Edge. We still provide a bundle which should work on IE11, but we are not running tests on it.</p>