signature: `PrehashSignature` is this trait still needed?

[carloskiki]

The derive feature of signature is removed in version 3.0.0, and I believe PrehashSignature was used as a helper to derive the Signer/Verifier traits. Now that derive is removed, do we still need this trait? IMHO it feels weird that this is implemented on signatures rather than the signing/verifying keys, because the associated Digest type can only be seen as a "preference". When DigestSigner is implemented, the user must provide the correct digest algorithm, which I think makes a better API.

Note: there was already a motion to remove the trait in RustCrypto/signatures#27.

[tarcieri]

All current usages would first need to be removed, e.g. from the ecdsa crate

[tarcieri]

Looks like this is safe to move ahead with now