This was used to create HTML elements for a shadow dom, but this
is now provided by the scoped-elements mixin via this.createScopedElement().

One way to handle this is enough, so remove this again.
All users have been ported already.

also adds two plural forms that were missing

It has new API which we want to use.

This got added in scoped-element 2.1.0 and is more or less the same
as getShadowRootDocument()

This is inspired by how lion web components handle this, and much cleaner.