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/_shortcodes.scss

@@ -31,10 +31,6 @@
     padding: $padding-16;
   }
 
-  .book-expand-expander {
-    display: none;
-  }
-
   input[type="checkbox"]:checked + .book-expand-content {
     display: block;
   }
@@ -60,10 +56,6 @@
     cursor: pointer;
   }
 
-  .book-tabs-radio {
-    display: none;
-  }
-
   .book-tabs-content {
     order: 999; //Move content blocks to the end
     width: 100%;