core/skeleton: drop /etc/securetty

securetty is supposed to restrict the terminals root is allowed to
login from. As it happens, login from busybox (w/ securetty support)
is actually enforcing use of securetty, while login from util-linux
is completely ignoring securetty altogether.

Remove securetty from our skeleton altogether and stop worrying about
it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>