use class="hidden" instead of creating new classes

Use class "hidden" instead of styled classes to ensure that input elments stay
hidden when the Content-Security-Policy header has "style-src 'self';"

assets/book.scss

@@ -54,11 +54,6 @@ img {
   vertical-align: baseline;
 }
 
-// hide menu checkbox with CSS
-#menu-control {
-  display: none;
-}
-
 aside nav ul {
   padding: 0;
   margin: 0;