Remove deprecated show_error_codes mypy setting

Mypy now shows error codes by default. It's unclear when this changed.
It doesn't seem to be mentioned in the changelog, and both the original
`show_error_codes` setting and the deprecation seem to be undocumented.

This commit will remove `show_error_codes`/`--show-error-codes`.

The inverse `hide_error_codes`/`--hide-error-codes` is still supported.

https://github.com/python/mypy/blob/master/CHANGELOG.md
python/mypy#13542
python/mypy#17083
https://mypy.readthedocs.io/en/stable/error_codes.html

SchemaStore/schemastore#3422
SchemaStore/schemastore#3467
SchemaStore/schemastore#3472
https://www.schemastore.org/partial-mypy.json

Author: br3ndonland

pyproject.toml

@@ -116,7 +116,6 @@ source = ["fastenv", "tests"]
 
 [tool.mypy]
 files = ["**/*.py"]
-show_error_codes = true
 strict = true
 
 [tool.pytest.ini_options]