attrs: remove fields type check

[ikonst]

Since python-attrs/attrs#890 (≥ 22.1.0) attrs.fields is typed to accept a protocol.
Since python-attrs/attrs#997 (≥ 22.2.0) attrs.has is a type-guard.

Support both by removing the explicit error reporting and letting it fall through to the type stub.

Fixes #15980.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect): typechecking got 1.10x slower (110.1s -> 120.5s)
(Performance measurements are based on a single noisy sample)

[ikonst]

@sobolevn this is along the same lines as #15962

btw, the reason why I'm not culling the same code (wrote it at the same time...) for attrs.replace is that its signature is still attrs.replace(obj: T, ...) where T is not bound, i.e. the typeshed offers no validation of the first arg being an attrs instance.

[sobolevn]

Thanks!

[hauntsaninja]

Thanks for this!

[Tinche]

@ikonst haha thanks for this, I had it on my mental checklist