[Refactor] Remove unnecessary fields in KZG verifier parameter structs

[chancharles92]

Currently, the SRS and KZG verifier params have fields g, h, and beta_h that can be removed as they can be retrieved from powers_of_g and powers_of_h. https://github.com/EspressoSystems/jellyfish/blob/kzg-multiproof-binyi/primitives/src/pcs/univariate_kzg/srs.rs#L18 https://github.com/EspressoSystems/jellyfish/blob/kzg-multiproof-binyi/primitives/src/pcs/univariate_kzg/srs.rs#L49 (P.S. It's better to expose methods only rather than fields in the future to enable easier maintenance.)

Referenced discussion: https://github.com/EspressoSystems/jellyfish/pull/406#discussion_r1389855533

(cc @alxiong @mrain @ggutoski , not a high-priority task, but we should do it sometime.)

[alxiong]

Yeah agree. We can always add convenient getters like VerifierParam::g() which returns powers_of_g[0].