system/skeleton: drop adm user from shadow

It has no counterpart in passwd so it's basically a NOP.

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

system/skeleton/etc/shadow

@@ -1,7 +1,6 @@
 root::10933:0:99999:7:::
 bin:*:10933:0:99999:7:::
 daemon:*:10933:0:99999:7:::
-adm:*:10933:0:99999:7:::
 lp:*:10933:0:99999:7:::
 sync:*:10933:0:99999:7:::
 shutdown:*:10933:0:99999:7:::