remove pthread_exit() from rust_start()

It was causing OS X Lion to hang and (according to Apple) is undefined
in any case

src/rt/rust.cpp

@@ -109,12 +109,6 @@ rust_start(uintptr_t main_fn, int argc, char **argv,
 
     free_env(env);
 
-#if !defined(__WIN32__)
-    // Don't take down the process if the main thread exits without an
-    // error.
-    if (!ret)
-        pthread_exit(NULL);
-#endif
     return ret;
 }