Remove unused header declaration

Summary: title

Test Plan: compile

Reviewers: sergei, bogdan

Reviewed By: bogdan

Differential Revision: https://phabricator.dev.yugabyte.com/D8715

Author: iSignal

src/yb/util/net/dns_resolver.cc

@@ -47,7 +47,6 @@
 DEFINE_int32(dns_num_resolver_threads, 1, "The number of threads to use for DNS resolution");
 TAG_FLAG(dns_num_resolver_threads, advanced);
 
-DECLARE_string(resolver_address_filter);
 using std::vector;
 
 namespace yb {