Remove unused function: dns_randfn_() in dns.c.

This patch unbreaks the current build after tor!369 landed.

See: https://bugs.torproject.org/tpo/core/tor/40371

src/feature/relay/dns.c

@@ -212,14 +212,6 @@ evdns_log_cb(int warn, const char *msg)
   tor_log(severity, LD_EXIT, "eventdns: %s", msg);
 }
 
-/** Helper: passed to eventdns.c as a callback so it can generate random
- * numbers for transaction IDs and 0x20-hack coding. */
-static void
-dns_randfn_(char *b, size_t n)
-{
-  crypto_rand(b,n);
-}
-
 /** Initialize the DNS subsystem; called by the OR process. */
 int
 dns_init(void)