[Distutils] Removing wheel signing features from the wheel library

After spending quite some time thinking about this, I've decided to cut out the wheel signature related features from the wheel codebase, unless there is significant resistance among the readers of this mailing list. For those not involved in the previous discussion, the reasoning is that the codebase can be significantly simplified by removing this rarely used feature whose practical value is questionable at best, given the lack of infrastructure for public key distribution.

If, as a result, there is a public outcry (which I sincerely doubt), I can always add it back to the next release.