Remove addHTMLElement from Bindings.conf.

We can just use the defaults for all of the HTML elements for now and
deal with the nonstandard behavior later. This fixes #2207.
auto merge of #2428 : zwarich/servo/remove-add-html-element, r=jdm

We can just use the defaults for all of the HTML elements for now and
deal with the nonstandard behavior later. This fixes #2207.