Don't create a default pidfile

Index: src/apps/relay/mainrelay.c
--- src/apps/relay/mainrelay.c.orig
+++ src/apps/relay/mainrelay.c
@@ -126,7 +126,7 @@ turn_params_t turn_params = {
 
     0, /* do_not_use_config_file */
 
-    "/var/run/turnserver.pid", /* pidfile */
+    "",                        /* pidfile */
     "",                        /* acme_redirect */
 
     ////////////////  Listener server /////////////////
