ecdsa: Remove implementations of PrehashSignature

[carloskiki]

As discussed in RustCrypto/traits#1917.

I kept the DigestAlgorithm trait because it is still used as a bound in some places, but it could be removed. For example:

impl<C, D> DigestSigner<D, Signature<C>> for SigningKey<C>
where
    C: EcdsaCurve + CurveArithmetic + DigestAlgorithm,
    D: Digest + FixedOutput,
    Scalar<C>: Invert<Output = CtOption<Scalar<C>>>,
    SignatureSize<C>: ArraySize,
{
    fn try_sign_digest(&self, msg_digest: D) -> Result<Signature<C>> {
        self.sign_prehash(&msg_digest.finalize_fixed())
    }

Here, C: DigestAlgorithm is used, but DigestSigner is implemented for any D, not for <C as DigestAlgorithm>::Digest. Removing this trait bound would thus have no effect.