File size calculating is borked entirely

It [uses the admin-ajax callback](https://github.com/humanmade/WP-Remote-WordPress-Plugin/blob/0001900e6ed3ecff5abe6ffaf29a52cf1981e3b3/wprp.backups.php#L203) I removed in #87 

Considering it was a public admin ajax endpoint though, I don't think we should restore it. We'll need to find another solution.

cc @willmot