Remove HTMLMediaElement.webkitPreservesPitch

It is not implemented, so remove it until there is a spec, at
which point it can be implemented without a prefix.

BUG=305554

Review URL: https://codereview.chromium.org/26679002

git-svn-id: svn://svn.chromium.org/blink/trunk@159246 bbb929c8-8fbe-4397-9dbb-9b2b20218538