Remove HTMLPlugInImageElement.

HTMLPlugInImageElement provides nothing. HTML{Applet,Embed,Object}Element
inherit from HTMLPugInElement directly.

BUG=309978
TEST=none; no behavior changes.
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/38533005

git-svn-id: svn://svn.chromium.org/blink/trunk@160416 bbb929c8-8fbe-4397-9dbb-9b2b20218538