Overriding `Future.onSuccess` in `FutureConvertersImpl`

[NthPortal]

FutureConvertersImpl.P overrides onSuccess here with a call to super. Is there an important reason for that, or can it be removed?

This comes up because onSuccess is deprecated, and may be removed in 2.13 (see scala/scala#6319).

[NthPortal]

If it can be safely removed, #100 removes it.