Remove <applet>

This removes support for the applet element from:

* document.all[name]
* document[name]
* window[name]

Furthermore:

* document.applets now returns an empty array
* Removed handling for applet elements in <iframe sandbox>
* Removed rendering rules for applet elements
* Removed the element itself and HTMLAppletElement
* Applet elements now use HTMLUnknownElement

The HTML parser is intentionally not changed.

Tests: web-platform-tests/wpt#6684.

Fixes #454.