DevTools: remove unused CSS.

See:
https://codereview.chromium.org/510743002

BUG=

Review URL: https://codereview.chromium.org/512113002

git-svn-id: svn://svn.chromium.org/blink/trunk@181037 bbb929c8-8fbe-4397-9dbb-9b2b20218538