Remove redundant DOING_AJAX check

DOING_AJAX is always going to be the same as is_admin(), as admin-ajax.php, which defines DOING_AJAX, is loaded in an admin context. 

Given that it's loading admin-facing functionality, it seems more idiomatically proper to only check is_admin().
Merge pull request #100 from JustinSainton/patch-1

Remove redundant DOING_AJAX check