The "data" parameter was meant to be used as an opaque type, but as
"forget" method needs to know its structure, stop pretending it's opaque.
This simplifies dispatch arguments a little bit.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2559 71dcc9de-d417-0410-9af5-da40c76e7ee4