Remove methods from RegisteredXxxProof types

[anorth]

Methods on these integers are handy, but they complicate making any structural changes without integrating those changes into every actors release branch. E.g. we will probably want to remove RegisteredSealProof.RegisteredWindowPoStProof() because there's no necessary bijection there, but all call sites in all actors release branches need to handle that.

If such methods were free functions, we'd have more flexibility to change without forcing lots of work or a major version upgrade of this repo.

[jennijuju]

is this still needed?

[anorth]

We still should do it IMO, but its importance depends on the versioning strategy for this repo. I'll defer to whoever has the vision for that @arajasek ?