Do not require public keys in libhsm

When a key-pair is generated by OpenDNSSEC, we should discard the public key object. There are several reasons to do this:

• The private key object contains all the information you need on a key, including the public components (CKA_PUBLIC_EXPONENT, CKA_MODULUS). The public key object is only there to facilitate extraction of public key
  material without having to log in to a token.

• Some HSMs limit the number of objects you can store on them as a licensing model (e.g. Safenet HSMs), and USB tokens can certainly only
  store a very limited number of objects, so not storing the public key object has a big benefit in both these cases

• Currently, libhsm searches for the whole key-pair, finding both the public and the private key object and matching them up. Cutting out the public key object would improve this because you no longer have to search for it.