Intent to unship: DOMRequest

[Gregory Pappas]

As of Firefox 124, I intend to turn off support for DOMRequest. This API is
non-standard and was never implemented by any other browser.

DOMRequest is a Promise-like interface that was implemented before Promises
existed. DOMRequest was used by a lot of APIs in Firefox OS.

Now that all APIs that returned a DOMRequest object have either been removed
or updated to return Promises instead, we can remove this non-standard
interface.

Bug: 1880615

Specification: Does not exist

Preference: dom.domrequest.enabled

[Kagami Rosylight]

Note that DOMRequest cannot be constructed by JS, and as mentioned it's never returned from any API, so the only impact here is that `window.DOMRequest` will now disappear.