Specialise: Get rid of obsolete `se_interesting` mechanism

Follow-up from "Specialising through specialised method calls (#19644 (closed))", !7956 (merged).

Simon writes:

Good! But you can now eliminate se_interesting entirely! Dictionaries have unfoldings, so interestingDict will see that.

NB: I think cloneIdBndr does not drop unfoldings, contrary to the comment

 * Local variables are cloned on the way down (to avoid clashes when
   we float dictionaries), and cloning drops the unfolding
   (cloneIdBndr).  Moreover, we make up some new bindings, and it's a

You'll need to revise/delete

• Note [Interesting dictionary arguments] -- still needed in some form.
• Note [Make the new dictionaries interesting] and
• Note [Keep the old dictionaries interesting] and

I think that the latter two are now automatically fine, because we are maintaining unfoldings. But worth checking.