Remove temporary binary compat scaffolding from AbstractPartiionFun.

These were only needed during the transition period when we removed
specialization on AnyRef from `AbstractPartialFunction`.

It's time to cast away the crutches.
Merge pull request #3347 from retronym/topic/abs-part-fun-manual-spec-no-more

Remove temporary binary compat scaffolding from AbstractPartiionFun.