hs: Remove the private key material from hs_descriptor.h
Basically, we need to remove the private keys from some data structure in `hs_descriptor.h`. Namely: ``` ed25519_keypair_t signing_kp; curve25519_keypair_t curve25519; ``` Those private keys should be in a dedicated data structure on the service side which we don't have yet so only put public keys there.
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