app: remove use of deprecated iframe frameborder property

The recommended way to do this is to use style sheets instead.

src/app/App.tsx

@@ -148,7 +148,6 @@ const Docs: React.VFC = () => {
             role="documentation"
             width="100%"
             height="100%"
-            frameBorder="none"
         />
     );
 };

src/app/app.scss

@@ -47,6 +47,10 @@
             position: absolute;
         }
     }
+
+    & iframe {
+        border: none;
+    }
 }
 
 .pb-app-body {