Remove Python::register_any

[davidhewitt]

#1020 is set to remove the last internal use case of register_any. IMO it adds complexity and now doesn't have a clear use case, should we remove this API (or mark deprecated)?

Originally posted by @davidhewitt in https://github.com/PyO3/pyo3/pull/1020/files

Quick github search suggests that @programmerjake is using register_any to allow conversion of &T for a special case:

https://github.com/programmerjake/algebraics/blob/fefb5a60922a6fa1d1dcb7bd0409c5a0a0e66515/src/python.rs#L30

I don't see any other uses on the whole of github 😄

[programmerjake]

feel free to remove it since you're doing a major version bump anyway, I'll be refactoring algebraics's API when I get around to updating it to PyO3 0.11 or later.

[kngwyu]

Thank you for the feedback!

[kngwyu]

Closed via #1020