generic_server: replace empty destructor with = default

This change improves code readability by explicitly marking the destructor as defaulted.

(cherry picked from commit 27b3d5b415)

generic_server.cc

@@ -276,10 +276,6 @@ server::server(const sstring& server_name, logging::logger& logger, config cfg)
 {
 }
 
-server::~server()
-{
-}
-
 future<> server::stop() {
     co_await shutdown();
     co_await std::exchange(_all_connections_stopped, make_ready_future<>());

generic_server.hh

@@ -130,7 +130,7 @@ private:
 public:
     server(const sstring& server_name, logging::logger& logger, config cfg);
 
-    virtual ~server();
+    virtual ~server() = default;
 
     // Makes sure listening sockets no longer generate new connections and aborts the
     // connected sockets, so that new requests are not served and existing requests don't