* Centralise the attachment deleting code into the Message class

* Centralise the attachment deleting code into the Message class
* Use this from the various places in the code where this was duplicated
* Fix the broken code in signout.php to cleanup lingering attachments
  by using this same function.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10387 7612ce4b-ef26-0410-bec9-ea0150e637f0