Remove mypy `show-error-codes` configuration which will become default

When the next Mypy release is released with:

- https://github.com/python/mypy/pull/13542
- https://github.com/python/mypy/issues/13685

Remove this line:

https://github.com/johnthagen/python-blueprint/blob/0d752df888a6cd0f9821e92091372d3c74c3090b/pyproject.toml#L80