Remove methods from IDL that don't do anything useful.
Move HTMLDocument members to Document.
Remove HTMLDocument.
Remove document::DocumentType (fixes #1730).
Remove DocumentTypeId.
auto merge of #1739 : Ms2ger/servo/document-cleanup, r=jdm

The specs currently have only Document; this may well change, but the split won't be along the lines we've implemented right now. In the meantime, this simplifies our code quite a bit.