system/skeleton: drop backup user

AFAIK no package uses it and even if it did it would belong there.
And it's not a standard user either.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

system/skeleton/etc/passwd

@@ -6,7 +6,6 @@ sync:x:4:100:sync:/bin:/bin/sync
 mail:x:8:8:mail:/var/spool/mail:/bin/sh
 proxy:x:13:13:proxy:/bin:/bin/sh
 www-data:x:33:33:www-data:/var/www:/bin/sh
-backup:x:34:34:backup:/var/backups:/bin/sh
 operator:x:37:37:Operator:/var:/bin/sh
 ftp:x:83:83:ftp:/home/ftp:/bin/sh
 nobody:x:99:99:nobody:/home:/bin/sh