[Distutils] Removing wheel signing features from the wheel library

[alex.gronholm at nextday.fi]

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.