Clean up use of Flake8 in GitHub CI (#46740)
Summary:
[Previously](https://github.com/pytorch/pytorch/runs/1293724033) Flake8 was run using `flake8-mypy`, which didn't change the actual lint output, and undesirably resulted in this noisy message being printed many times:
```
/opt/hostedtoolcache/Python/3.9.0/x64/lib/python3.9/site-packages is in the MYPYPATH. Please remove it.
See https://mypy.readthedocs.io/en/latest/running_mypy.html#how-mypy-handles-imports for more info
```
Since `mypy` is already run in other test scripts, this PR simply removes it from the Flake8 setup. This PR also removes the `--exit-zero` flag from Flake8, because currently Flake8 gives no error output, so it would be valuable to know if it ever does happen to return error output.
(This doesn't strike me as a perfect solution since now it's a bit harder to reproduce the Flake8 behavior when running locally with `flake8-mypy` installed, but it's the easiest way to fix it in CI specifically.)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46740
Reviewed By: janeyx99
Differential Revision: D24487904
Pulled By: samestep
fbshipit-source-id: d534fdeb18e32d3bc61406462c1cf955080a688f
