cool-cats: only fetch the ids to limit footprint

We don't really care much about the returned; all we want is the count
so there's no point in fetching all the category data.

inc/template-tags.php

@@ -103,6 +103,7 @@ function _s_categorized_blog() {
 		// Create an array of all the categories that are attached to posts.
 		$all_the_cool_cats = get_categories( array(
 			'hide_empty' => 1,
+			'fields' => 'ids',
 		) );
 
 		// Count the number of categories that are attached to the posts.