Remove native windows RenderTheme code

Now that we're committed to Aura, there's no need to keep this dead code around.

R=jamesr@chromium.org
BUG=330735

Review URL: https://codereview.chromium.org/145353011

git-svn-id: svn://svn.chromium.org/blink/trunk@166074 bbb929c8-8fbe-4397-9dbb-9b2b20218538