`namespace.delete` should not preserve history

[pchiusano]

@SystemFw ran into this recently.

We changed move.namespace to not preserve the history, and I think the same reasoning applies here (I can't find the ticket at the moment). For instance:

.proj> pull user1.public.blah.releases.wrongVersion lib.blah
-- oops, pulled the wrong version
.proj> delete.namespace lib.blah
.proj> pull somewhere.public.blah.latest lib.blah

... will silently do something strange.

Let's change namespace.delete to work the same as namespace.move, obliterating the history.