In `WP_MS_Users_List_Table::get_views()`, remove dead code:

* `$wp_roles` doesn't need to be imported.
* `$current_role` is set and never used.

See #27882.


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git-svn-id: http://core.svn.wordpress.org/trunk@28123 1a063a9b-81f0-0310-95a4-ce76da25c4cd