Remove lines about timestamp in Rekor lib.rs

[lkatalin]

They are outdated.

https://github.com/sigstore/sigstore-rs/blob/371b22af9d3ae49a5bcf7fd0bca963c75e39d8f5/src/rekor/lib.rs#L82-L83 https://github.com/sigstore/sigstore-rs/blob/371b22af9d3ae49a5bcf7fd0bca963c75e39d8f5/src/lib.rs#L203-L204