nghttpx: Remove unused EMPTY_STRING

src/shrpx_config.cc

@@ -69,8 +69,6 @@ Config *mod_config() { return config; }
 
 void create_config() { config = new Config(); }
 
-std::string EMPTY_STRING;
-
 TicketKeys::~TicketKeys() {
   /* Erase keys from memory */
   for (auto &key : keys) {

src/shrpx_config.h

@@ -255,10 +255,6 @@ enum shrpx_forwarded_node_type {
   FORWARDED_NODE_IP,
 };
 
-// Used inside function if it has to return const reference to empty
-// string without defining empty string each time.
-extern std::string EMPTY_STRING;
-
 struct AltSvc {
   AltSvc() : port(0) {}