Remove "onhide" from obsolete attributes

Fixes whatwg#2860. "onhide" was never an attribute in the first place. This
reference to it was erroneously introduced in
e7e8c88.

Author: a1626

source

@@ -113226,7 +113226,6 @@ if (s = prompt('What is your name?')) {
    <dt><dfn><code data-x="attr-menu-label">label</code></dfn> on <code>menu</code> elements</dt>
    <dt><dfn><code data-x="attr-contextmenu">contextmenu</code></dfn> on all elements</dt>
    <dt><dfn><code data-x="handler-onshow">onshow</code></dfn> on all elements</dt>
-   <dt><dfn><code data-x="handler-onhide">onhide</code></dfn> on all elements</dt>
    <dd><p>To implement a custom context menu, use script to handle the <code
    data-x="event-contextmenu">contextmenu</code> event.</p></dd>