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>