remove HTMLAnchorElement::PrefetchEventHandler

The measurement code is always disabled now. Removing it from the codebase.

BUG=386888

Review URL: https://codereview.chromium.org/344133002

git-svn-id: svn://svn.chromium.org/blink/trunk@176575 bbb929c8-8fbe-4397-9dbb-9b2b20218538