hs: Remove the introduction point private key material from hs_descriptor.h
We need to change the curve25519_keypair_t
from hs_desc_intro_point_t
to a public key only because both client and service will use that field.
We need to change the curve25519_keypair_t
from hs_desc_intro_point_t
to a public key only because both client and service will use that field.