resolved: drop unnecessary comparison (#6220)

mtu is always greater than UDP_PACKET_HEADER_SIZE at this point.
Pointed out by Benjamin Robin.

src/resolve/resolved-dns-packet.c

@@ -57,7 +57,7 @@ int dns_packet_new(DnsPacket **ret, DnsProtocol protocol, size_t mtu) {
         if (mtu < UDP_PACKET_HEADER_SIZE)
                 a = DNS_PACKET_SIZE_START;
         else
-                a = MAX(mtu, DNS_PACKET_HEADER_SIZE);
+                a = mtu;
 
         /* round up to next page size */
         a = PAGE_ALIGN(ALIGN(sizeof(DnsPacket)) + a) - ALIGN(sizeof(DnsPacket));