chore: Remove unused [tool.black] and [tool.pycln] config (#2891)

# Rationale for this change

This PR removes [tool.black] from pyproject.toml. This is an unused
config since formatting now lives in
[ruff.toml](https://github.com/apache/iceberg-python/blob/main/ruff.toml).
Also, updates contributing.md to reference ruff instead of black.

This PR removes dead config from pyproject.toml:
- [tool.black] - replaced by ruff-format #127
- [tool.pycln] - replaced by ruff #1485 

Also updates contributing.md to reference ruff instead of black.

## Are these changes tested?

Yes

## Are there any user-facing changes?

No