[dev-util] Remove reference to /root/.forget_usernames from crdev
[dev-util] Remove reference to /root/.forget_usernames from crdev

/root/.forget_usernames is going to be removed from test images.

BUG=chromium-os:31807
TEST=suite:bvt

Change-Id: I41dda4ebfb6ab9217a1eba877ee1d0172974c62a
Reviewed-on: https://gerrit.chromium.org/gerrit/27409
Commit-Ready: Nirnimesh <25e32f8a023b4789ee4f4636c1d5b28f8624cb3b@chromium.org>
Reviewed-by: Nirnimesh <25e32f8a023b4789ee4f4636c1d5b28f8624cb3b@chromium.org>
Tested-by: Nirnimesh <25e32f8a023b4789ee4f4636c1d5b28f8624cb3b@chromium.org>

mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util