DevTools: remove unused CSS.

See: https://codereview.chromium.org/449893002

BUG=
TBR=vsevik@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/471553003

git-svn-id: svn://svn.chromium.org/blink/trunk@180176 bbb929c8-8fbe-4397-9dbb-9b2b20218538