As #13479 shows, there are corner cases where the simplifier decides to
not eta-expand a function as much as its call arity would suggest, but
instead transforms the code that the call arity annotation becomes a
lie.

As the call arity information is only meant to be used by the
immediatelly following simplifier run, it makes sense to simply zap the
information there.

Differential Revision: https://phabricator.haskell.org/D3390