Temporary attachments aren't deleted when the session is destroyed

Steps to reproduce:

• Click edit (with the WYSIWYG editor) and upload an image
• Open a new tab without closing the editor and log out

Expected result:

The uploaded attachment is deleted from the file system as the session has been destroyed during logout.

Actual result:

The file in xwiki-temp/attachment-cache/ still exists (needs to be checked on the filesystem).