Remove menuitem special-casing

[domenic]

We ended up removing it from the spec a while back: whatwg/html#2319

But it looks like there is still code here for it, based on a brief skim.

[inikulin]

Yeah, it was already removed in the Parse Error WG fork. It will be merged once we accomplish tree construction errors. Is it hits you in jsdom at the moment somehow and you need a quick fix? Or you can wait till Parse Error WG fork will be merged into upstream?

[domenic]

No, no rush, I was just reminded in whatwg/html#2730 (comment) that we never filed a parse5 bug when removing it, and doing so is a good idea.

[inikulin]

Great then. Thanks for the report!

[inikulin]

I'm actually going to use upstream html5lib tests for daily CI builds in the future, so once something will change in the spec and, thus, hopefully, in tests we will get notifications about it automatically.

[inikulin]

Changes from parse error working group's fork has been recently merged, so closing this one.