Remove cipher suite and version from Secret

[raphaelrobert]

Fixes #1511.

This PR removes the ciphersuite and protocol version from the Secret struct:

pub(crate) struct Secret {
    pub(in crate::ciphersuite) ciphersuite: Ciphersuite,
    pub(in crate::ciphersuite) value: SecretVLBytes,
    pub(in crate::ciphersuite) mls_version: ProtocolVersion,
}

↓

pub(crate) struct Secret {
    pub(in crate::ciphersuite) value: SecretVLBytes,
}

To make the review easy, this is the only change. The rest of the PR is churn. There's quite a bit of it, since the secrets were used to carry the ciphersuite which now has to be passed manually. Conversely, the protocol version no longer has to passed in quite a few functions.

[kkohbrok]

Looks good to me with one nit.