In `wp-includes/revision.php` - remove dead code:

* In `wp_get_post_autosave()`, `break` is unreachable after `return`
* In `_wp_put_post_revision()`, `$post_id` is set then never used.

See #27882.


git-svn-id: https://develop.svn.wordpress.org/trunk@28334 602fd350-edb4-49c9-b593-d223f7449a82