[Sheriff-o-Matic] Remove back button.

Sheriff-o-Matic supports proper history navigation (see r181089), so no
need for explicit back button in UI any more.

BUG=409979
R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/617633002

git-svn-id: svn://svn.chromium.org/blink/trunk@182881 bbb929c8-8fbe-4397-9dbb-9b2b20218538