netcmd: silo command remove combined --policy which set all 3

netcmd: silo command remove combined --policy which set all 3

doesn't make much sense to set all 3 to the same policy, user authentication policy, service authentication policy, computer authentication policy

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>