Deprecate keys-gen

[jdknives]

The keys-gen cmd is duplicating logic, albeit minimal logic, that we already have in https://github.com/skycoin/skycoin/tree/develop/cmd/address_gen apparently. We need to verify that address-gen can generate keypairs and do so with a seed being inputted and if it can do that, we should update documentation to point users to address-gen instead and explain how to setup and use it.