apply filter on $sizes earlier

Hooking *intermediate_image_sizes_advanced* directly in `ajax_thumbnail_rebuild_get_sizes()` makes the management page's list display consistent info (see L.178) in respect to what is processed by `wp_generate_attachment_metadata_custom()` – where the filter is currently applied.

My user case: I needed to hook *intermediate_image_sizes_advanced* to remove Wordpress' default image sizes as in [this example](http://www.paulund.co.uk/remove-default-wordpress-image-sizes). Before, in the plugin's management page, default image sizes were still shown in the list although not rebuilt.