[FIX] website_slides: still another fix of computed fields

This commit fixes 2 issues coming from the recent ORM changes: records are
new records in onchange and compute methods must assign a value for all records.

In an onchange method, self.ids is a list of integer (real records)
whereas self.id is a NewId (virtual record)

Here a result dict was built using the reals ids as keys but later NewIds
are used as keys. Solution is to use record._origin to access the real record.

Moreover, the two dicts `result` and `type_stats` are the same. Assigning one
into the other is useless.

Task 2067872 (eLearning internal testing)
PR #36756