net/dns: Fix removal of DNS server

net/dns: Fix removal of DNS server

When deleting the DNS server, we get the following error message:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
grub> net_del_dns 192.168.122.1
error: ../../grub-core/net/dns.c:646:no DNS reply received.
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

This happens because the implementation is broken, it does a "add"
internally instead of a "delete".

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>