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

* Extracting `$this->_args` is unnecessary since none of the variables produced are present in the method.
* `total_items` and `total_pages` can be read directly from `$this->_pagination_args`

See #22400.

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git-svn-id: http://core.svn.wordpress.org/trunk@28215 1a063a9b-81f0-0310-95a4-ce76da25c4cd