Delete pybundles

This removes the bundle functionality that was deprecated in 1.4. Originally there was some talk about waiting to remove this until we split it out into it's own command and possibly have some extension support. I don't think it makes sense to wait because none of us are motivated to actually maintain it and I don't see why splitting it out into a second repository is going to change that. Additionally even if we do split it out we (or someone else) can pull it from the VCS history so keeping it around in the branch isn't winning us much. Finally we don't need extension support because there isn't much difference between typing ``pip bundle`` vs ``pip-bundle`` and anyone can provide a ``pip-bundle`` command without any help from us.

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