Remove obsolete HitTestResult.targetNode

This has been an alias for innerNode with a "remove this" FIXME for some
time.  The cleanup is pretty easy.

BUG=

Review URL: https://codereview.chromium.org/476303002

git-svn-id: svn://svn.chromium.org/blink/trunk@181786 bbb929c8-8fbe-4397-9dbb-9b2b20218538