CLEANUP: dns: remove obsolete macro DNS_MAX_IP_REC

Since a8c6db8d2d97629b2734c1d2be0860b6b11e5709, this macro is not used
anymore and can be safely removed.

Backport status: 1.8
diff --git a/src/dns.c b/src/dns.c
index 8d6a6d6..8ae183f 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -949,7 +949,6 @@
  * For both cases above, dns_validate_dns_response is required
  * returns one of the DNS_UPD_* code
  */
-#define DNS_MAX_IP_REC 20
 int dns_get_ip_from_response(struct dns_response_packet *dns_p,
                              struct dns_options *dns_opts, void *currentip,
                              short currentip_sin_family,