digest: remove `VariableOutput`?

[newpavlov]

The VariableOutput trait allows users to select hash output size at runtime. Initially it was added for completeness sake, but it does not look like it's used in practice.

Arguably, all reasonable use cases can be covered by output size selected at compile time, so it may be worth to completely remove the trait and associated types from implementation traits.

[tarcieri]

I'm not personally using it so I don't have a strong opinion. It seems useful for blake2, though I notice the blake3 crate doesn't use it.

[newpavlov]

We have other variable output hashes in addition to blake2 (e.g. groestl and kupyna).

The question whether we need Blake2sVar in addition to Blake2s. Judging by the GitHub search results, the answer seems to be no.