remove xvfb from sheriff-o-matic makefile

BUG=404414
R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/483773004

git-svn-id: svn://svn.chromium.org/blink/trunk@180484 bbb929c8-8fbe-4397-9dbb-9b2b20218538