Remove `<rb>` element. Not in html5.

I already have one change that uses my latest change (#50, thx for that merge):
While some [old draft](http://www.w3.org/TR/1999/WD-ruby-19990322/) does mention `<rb>` elements, it is *not* in the  html5 [`<ruby>` elements](http://www.w3.org/TR/html-markup/ruby.html) description.
(This commit also brings the comment in line with #50.)