Fixes #1392 Move post(s) mod tool not changing 'new' thread's starter

Fixes #1392 
Isn't `$post['threadvisible']` the same as `$new_thread['visible']`? If so this `if` is useless too:
```php
if($post['visible'] != $newthread['visible'])
[...]
    if($post['threadvisible'] == 1 && $post['visible'] == 1)
[...]
```