Remove strange parentElement() code from HitTestResult

This code doesn't make any sense, why would the parentElement() be in
the document if the node wasn't?

It came from http://src.chromium.org/viewvc/blink?view=revision&revision=23994
7 years ago when the HitTestResult was actually stored in the Event, but
even then it doesn't seem to make sense.

Review URL: https://codereview.chromium.org/394013003

git-svn-id: svn://svn.chromium.org/blink/trunk@178225 bbb929c8-8fbe-4397-9dbb-9b2b20218538