Delete unused Windows UA string logic

We don't use this code to compute the UA string on Windows.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/25756004

git-svn-id: svn://svn.chromium.org/blink/trunk@158781 bbb929c8-8fbe-4397-9dbb-9b2b20218538