Remove `language_version` for pre-commit (#2430)

Remove `language_version` for pre-commit (#2430)

* Remove `language_version` for pre-commit

At my company, we set the Python version in `default_language_version`
in each repo's `.pre-commit-config.yaml`,
so that all hooks are running with the same Python version.

However, this currently doesn't work for black,
as the `language_version` specified here
in the upstream `.pre-commit-hooks.yaml` takes precedence.
Currently, this requires us to manually set `language_version`
specifically for black,
duplicating the value from `default_language_version`.
The failure mode otherwise is subtle -
black works most of the time,
but try to add a walrus operator and it suddenly breaks!

Given that black's `setup.py` already has `python_requires>=3.6.2`,
specifying that `python3` must be used here isn't needed
as folks inadvertently using Python 2 will get hook-install-time failures anyways.
Remove the `language_version` from these upstream hook configs
so that users of black are able to use `default_language_version`
and have it apply to all their hooks, black included.

Example `.pre-commit-config.yaml` before:
```
default_language_version:
  python: python3.8
repos:
-   repo: https://github.com/psf/black
    rev: 21.7b0
    hooks:
    -   id: black
        language_version: python3.8
```

After:
```
default_language_version:
  python: python3.8
repos:
-   repo: https://github.com/psf/black
    rev: 21.7b0
    hooks:
    -   id: black
```

* Add changelog entry