fix(server): remove invalid recv() call on listen socket

The recv() drain loop on listen_socket was incorrect - listen sockets
don't have data to read. This caused Valgrind to report 'points to
unaddressable byte(s)' errors during functional tests.

Replaced magic number 2 with SHUT_RDWR for clarity.

src/server/server.c

@@ -477,8 +477,7 @@ _shutdown(void)
 
     parser_close();
 
-    shutdown(listen_socket, 2);
-    char _discard; while (recv(listen_socket, &_discard, 1, 0) > 0) {}
+    shutdown(listen_socket, SHUT_RDWR);
     close(listen_socket);
 
     prime_free_all();