Cleanup after deleting entities

Problem/Motivation

I noticed that ACL module only deletes records from 'acl_node' when a node is deleted. This can lead to a lot of orphaned data being left in the 'acl' and 'acl_user' db tables. Do we have a reason to keep data in those other tables? To me it seems that when the node was deleted, all ACL related data should be irrelevant and therefore deleted as well.

I can prepare a patch for this in case we reach an agreement here.