Slim Cardano.Api.Certificate.Internal to pool-only definitions

Strip the deprecated legacy `Certificate era` GADT, its
`ShelleyRelatedCertificate`/`ConwayCertificate` constructors, every
`make*Certificate` constructor function, every `*Requirements` GADT,
`selectStakeCredentialWitness`, `toShelleyCertificate`,
`fromShelleyCertificate`, `isDRepRegOrUpdateCert`, and the legacy
`AnchorDataFromCertificateError` + `getAnchorDataFromCertificate`. The
module is now responsible only for the (non-deprecated) pool parameter
types and their conversion helpers.

Update consumers:

- `Sign.hs` re-exports `AsTxId` by importing it directly from
  `Cardano.Api.Tx.Internal.TxIn` (the previous re-export chain went
  through the legacy `AsType (AsCertificate, AsTxId)` in Internal). The
  `-Wno-deprecations` pragma is dropped because no deprecated symbol
  remains.
- `Body.hs` and `Certificate/Internal/OperationalCertificate.hs` drop
  their `Cardano.Api.Certificate.Internal` imports, which became unused
  once the legacy surface was removed.

Author: Jimbo4350