Clean up the orphan or unused h5p export files

As a consequence of MDL-66769 where we have developed a task to clean up the orphan or unuse H5P content when some activity/resource was been removed, we need to remove the unused h5p export files which are linked to system context.

In order to remove these files, we have to be sure that we could identify in the right way. For instance, taking a look at this function (get_export_info_from_context_id) we could figure out how to do it.

This is how is the filename is built:

$filename = "{$slug}{$content['id']}.h5p";