Remove media volume slider mute button.

Chromium's media controls implementation doesn't make use of this button.

BUG=337015
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/146173003

git-svn-id: svn://svn.chromium.org/blink/trunk@165777 bbb929c8-8fbe-4397-9dbb-9b2b20218538