[css-position-4] redundant document argument for top layer manipulation algorithms

[foolip]

https://drafts.csswg.org/css-position-4/#top-manip

This sections contains algorithms like "add an element to the top layer, given a Document doc and an Element el". The call sites for these in whatwg/fullscreen#223 and whatwg/html#9093 always pass in the element's node document as the first argument. In HTML there is a document variable handy so the invocation looks OK, but in Fullscreen we'll have invocations like this:

Request removal from the top layer given element's node document and element

By having only element as the argument, the call sites will be simpler, and there's no question about whether the document can be some other document.

cc @josepharhar @mfreed7 @tabatkins @nt1m @annevk

[josepharhar]

I am ok with removing the document parameter. Anne pointed this out in the HTML PR here: whatwg/html#9093 (comment)