Stop calling evdns_set_random_bytes_fn()

This function has been a no-op since Libevent 2.0.4-alpha, when
libevent got an arc4random() implementation.  Libevent has finally
removed it, which will break our compilation unless we stop calling
it.  (This is currently breaking compilation in OSS-fuzz.)

Closes #40371.

changes/bug40371

+  o Minor bugfixes (compatibility):
+    - Fix compatibility with the most recent Libevent versions, which
+      no longer have an evdns_set_random_bytes() function.  Because
+      this function has been a no-op since Libevent 2.0.4-alpha,
+      it is safe for us to just stop calling it.  Fixes bug 40371;
+      bugfix on 0.2.1.7-alpha.

src/feature/relay/dns.c

@@ -224,7 +224,6 @@ int
 dns_init(void)
 {
   init_cache_map();
-  evdns_set_random_bytes_fn(dns_randfn_);
   if (server_mode(get_options())) {
     int r = configure_nameservers(1);
     return r;