AthAllocators+GeneratorObjectsTPCnv: Change the DataPool cleanup template argument from a function to a functional.

DataPool has an option to call a cleanup function when an object is returned to the pool. This was done by adding the cleanup function pointer to DataPool. Templating on a function pointer is allowed, though a little unusual. However, with gcc15, i was seeing compilation failures related to partial specialization of functions with DataPool arguments with the cleanup argument defaulted.

I'm not sure that this isn't a gcc bug. However, changing from a function to a functional makes the problem go away, and that is anyway a more typical design.