system/skeleton: finalize removal of ftp user

In commit 3dde19e5, the ftp user was
removed from /etc/passwd, /etc/group and /home in the skeleton, but
the corresponding entry was not removed from /etc/shadow. This commit
fixes that.

Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>