simplify hashing

[jku]

Currently the hash module in securesystemslib supports multiple hash libraries:

• This seems bonkers to me and I've never seen anyone use this functionality
• It also makes type annotating the module a pain: this is pretty much the reason why securesystemslib is not yet marked as annotated (there are other unannotated modules but they are not as core the the library as hash is)

I think we can keep providing most of the API, but remove the cryptography support (from hashes module) and use only hashlib.

[jku]

https://github.com/jku/securesystemslib/tree/hash-simplify very quick hack