Unnecessary function: delete_files_fn

[Luke Nelson]

The function delete_files_fn( $target ) is unnecessary because its functionality can already be completed by $wp_filesystem->delete()

The function can be found at simpods-mvc/mvc/admin/models/pods_fields/podsfield_simpodspodsfile.php:754

It should be removed and any references to it should be replaced by WP filesystem.

[Jamin Xu]

We now use $wp_filesystem to do the job.