Missing method `get_guest_author_thumbnail` is throwing a notice and fatal

[this line](https://github.com/Automattic/Co-Authors-Plus/blob/ead0cb739646aeedba460df0dab23a5bf9f5bb99/template-tags.php#L514), introduced in ead0cb7396, is trying to use a method that does not exist anywhere in the codebase. This can cause a PHP notice on new post screens and a fatal error when listing authors.

It almost seems like there was a missed commit somewhere? I'm avoiding a PR right now as I'm not entirely sure what the exact intent was.