Eliminate use of `extract()` in `WP_Post_Comments_List_Table::display()`. The method only needs to read `singular` from `$this->_args`. Extraction is unnecessary.

Eliminate use of `extract()` in `WP_Comments_List_Table::display()`. The method uses no variables. Extraction can be completely removed.

See #22400.

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git-svn-id: http://core.svn.wordpress.org/trunk@28213 1a063a9b-81f0-0310-95a4-ce76da25c4cd