feat(identity): remove deprecated items

[tcoratger]

Description

Related: #3647.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Are all of those definitely released already? I remember merging a PR here after we opened the merge window for the breaking changes.

[tcoratger]

Are all of those definitely released already? I remember merging a PR here after we opened the merge window for the breaking changes.

@thomaseizinger I think any items that were flagged as deprecated in the identity folder may be removed from the codebase.

Indeed, I remembered this comment where I had eliminated in #3866 some residual calls to deprecated functions. It therefore made sense to me to eliminate all the remaining residues, especially since each time these are functions whose execution is guaranteed in exactly the same way by another updated function, what do you think?

[thomaseizinger]

Confirmed that all deprecations are listed on https://docs.rs/libp2p-identity/0.1.2/libp2p_identity/index.html and thus can be removed with this release.