doc: remove all scrollbar styling

Returns the doc custom scrollbar to native style.

Fixes: https://github.com/nodejs/node/issues/6443
PR-URL: https://github.com/nodejs/node/pull/6479
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>

doc/api_assets/style.css

@@ -55,35 +55,6 @@ em code {
   font-style: normal;
 }
 
-::-webkit-scrollbar {
-  -webkit-appearance: none;
-}
-
-::-webkit-scrollbar-button {
-  display: none;
-}
-
-::-webkit-scrollbar-track {
-  background: #fff;
-}
-
-::-webkit-scrollbar-thumb {
-  min-height: 48px;
-  background: #d2d2d2;
-  background-clip: padding-box;
-  border: 3px solid #fff;
-  border-radius: 5px;
-}
-
-::-webkit-scrollbar-thumb:active {
-  background: #888;
-  border-width: 2px;
-}
-
-::-webkit-scrollbar {
-  width: 10px;
-}
-
 #changelog #gtoc {
   display: none;
 }