EntityManager#remove() works only on entities which are managed in the current transaction/context. In your case, you’re retrieving the entity in an earlier transaction, storing it in the HTTP session and then attempting to remove it in a different transaction/context. This just won’t work.
You need to check if the entity is managed by EntityManager#contains() and if not, then make it managed it EntityManager#merge().
Basically, the delete() method of your business service class should look like this:
em.remove(em.contains(entity) ? entity : em.merge(entity));