system/skeleton: drop proxy 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

@@ -4,7 +4,6 @@ bin:x:2:2:bin:/bin:/bin/sh
 sys:x:3:3:sys:/dev:/bin/sh
 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
 operator:x:37:37:Operator:/var:/bin/sh
 ftp:x:83:83:ftp:/home/ftp:/bin/sh