Remove left-overs from -webkit-marquee-speed removal.

The slow, normal, and fast keywords, and the seconds unit, are not used by
the presentational style for the marquee element.

R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/457633002

git-svn-id: svn://svn.chromium.org/blink/trunk@179896 bbb929c8-8fbe-4397-9dbb-9b2b20218538