Remove `ForkName` from witness

[roynalnaruto]

After #164 is merged, we will no longer require fork_name: ForkName in the witness. Instead, the hardfork can be derived from the version byte, which is available in the witness.

This also means that the implementation of From<Witness> for LegacyWitness will change slightly:

impl From<ChunkWitness> for LegacyChunkWitness {
    fn from(value: ChunkWitness) -> Self {
        let version = Version::from(value.version);
        Self {
            /* ... */
            fork_name: version.fork,
            /* ... */
        }
    }
}