dns-manager: remove an incorrect and unnecessary line

gcc 4.8.0 has a new warning that triggers on

    static void
    compute_hash (NMDnsManager *self, guint8 buffer[HASH_LEN])
    {
        ...
        memset (buffer, 0, sizeof (buffer));
        ...
    }

because "sizeof (buffer)" is *not* HASH_LEN, it's sizeof(guint8*). The
memset() was not necessary anyway since the g_checksum_get_digest()
after it will always end up filling in buffer, so just remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=697041

src/dns-manager/nm-dns-manager.c

@@ -561,7 +561,6 @@ compute_hash (NMDnsManager *self, guint8 buffer[HASH_LEN])
 			nm_ip6_config_hash (NM_IP6_CONFIG (iter->data), sum, TRUE);
 	}
 
-	memset (buffer, 0, sizeof (buffer));
 	g_checksum_get_digest (sum, buffer, &len);
 	g_checksum_free (sum);
 }