Remove user "default"

User "default" with no password has been around for long time, but not
used within buildroot. Since we now have BR2_ROOTFS_USERS_TABLES it is
no longer needed.

Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>