windows: remove unneeded check

Cheking if the loop is alive is covered in the while loop afterwards.
Also, the stop flag will be cleared if necessary, which didn't happen
before this patch.

src/win/core.c

@@ -272,9 +272,6 @@ int uv_run(uv_loop_t *loop, uv_run_mode mode) {
   else
     poll = &uv_poll;
 
-  if (!uv__loop_alive(loop))
-    return 0;
-
   r = uv__loop_alive(loop);
   while (r != 0 && loop->stop_flag == 0) {
     uv_update_time(loop);