Remove dead content sniffing code

Removed ContentSniffingPolicy and all code related to content sniffing since they are unused.

BUG=409631

Review URL: https://codereview.chromium.org/531553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181260 bbb929c8-8fbe-4397-9dbb-9b2b20218538