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.

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git-svn-id: http://core.svn.wordpress.org/trunk@28217 1a063a9b-81f0-0310-95a4-ce76da25c4cd