Remove deprecated `instanceof`

[IanDelMar]

I also removed dealing with the $count argument which was deprecated in WP 4.5. In WP 4.5 also the position of the $args argument changed. Given we use 'get_terms' => 0, we're not supporting WP versions lower than 4.5. Hence $count had no effect.

Travis should be going green again soon. This was the last extension with deprecation notices.

Part of #149

[szepeviktor]

Travis should be going green again soon

You did it Ian!

[herndlm]

Great work!