Remove remained code of CSS Custom Filters.

This is follow-up CL for http://crrev.com/132873003

BUG=N/A

Review URL: https://codereview.chromium.org/228653002

git-svn-id: svn://svn.chromium.org/blink/trunk@171128 bbb929c8-8fbe-4397-9dbb-9b2b20218538