DevTools: remove unused css.

See:
https://codereview.chromium.org/519653002

NOTRY=true

Review URL: https://codereview.chromium.org/541513005

git-svn-id: svn://svn.chromium.org/blink/trunk@181370 bbb929c8-8fbe-4397-9dbb-9b2b20218538