Remove dead code from HTMLElement::parseAttribute

Element::parseAttribute does nothing so there's no reason to have a
special case to call it for id, class and style attributes.

Review URL: https://codereview.chromium.org/236363005

git-svn-id: svn://svn.chromium.org/blink/trunk@171431 bbb929c8-8fbe-4397-9dbb-9b2b20218538