Remove obsolete code from `WP_Users_List_Table::get_sortable_columns().

Sorting by post count was disabled in [17024].

Props jdgrimes.
See #15861.

git-svn-id: https://develop.svn.wordpress.org/trunk@35110 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-users-list-table.php

@@ -333,9 +333,6 @@ class WP_Users_List_Table extends WP_List_Table {
 			'email'    => 'email',
 		);
 
-		if ( $this->is_site_users )
-			unset( $c['posts'] );
-
 		return $c;
 	}