Saving a draft does not delete orphaned attachments from the server

This is not relevant if PHPBB3-16193 ever gets finished, but right now whenever a draft is saved, any attachments which were uploaded for the draft end up permanently orphaned instead of being cleaned from the server. A simple call to delete the orphaned attachments when a draft is saved would stop polluting the server with orphan attachments which are guaranteed not to be used again. See attached patch.

Reproduction:

1. Create post
2. Add attachment
3. Save as draft

Expected: Attachments are deleted from the server
Actual: Unrecoverable attachments are left on the server