Eliminate use of `extract()` in `WP_List_Table::pagination()`:

* Set variables instead of extracting `$this->_pagination_args`
* Weird: `$infinite_scroll` is only presently set in subclasses of `WP_List_Table` and never initialized otherwise. 		

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28389 602fd350-edb4-49c9-b593-d223f7449a82