Remove unused functions: WebKit::toHTMLLinkElement, WebKit::toHTMLMetaElement, and WebKit::toHTMLOptionElement.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/14246002

git-svn-id: svn://svn.chromium.org/blink/trunk@148336 bbb929c8-8fbe-4397-9dbb-9b2b20218538