We accidentally haven't been using $TMUX to work out the session for a while

and in fact it is less useful that using the client ttyname. So don't bother
and don't pass it from the client. If we need it in future it is in c->environ.

client.c

@@ -284,10 +284,6 @@ client_main(int argc, char **argv, int flags)
 			size += strlen(argv[i]) + 1;
 		data = xmalloc((sizeof *data) + size);
 
-		/* Fill in command line arguments. */
-		data->pid = environ_pid;
-		data->session_id = environ_session_id;
-
 		/* Prepare command for server. */
 		data->argc = argc;
 		if (cmd_pack_argv(argc, argv, (char*)(data + 1), size) != 0) {