Fix docs.rs build for datafusion-proto (hopefully)

[alamb]

Which issue does this PR close?

Closes #10163

Rationale for this change

Docs.rs is broken failed for https://docs.rs/crate/datafusion-proto/latest

The build has a log https://docs.rs/crate/datafusion-proto/37.0.0/builds/1179419 shows the error that the generated module isn't working

What changes are included in this PR?

Remove cfg to get working again (maybe)

Are these changes tested?

No -- I don't know how to test them. In the other hands the docs are broken so this isn't going to make them any more broken 🤷

Are there any user-facing changes?

[alamb]

I think this was added in #3580 but now that the code is vendored (doesn't require running protoc at compile time) I think it should work

However, I don't really understand why it stopped working in the first place 🤔

[phillipleblanc]

The last successful docs.rs build was DF 35 - I took a look at the commits in the proto directory between 35 and 36 and couldn't see anything obvious that would have caused this to fail.

Agree that we should give this a shot and see what happens in #10217

[leoyvens]

A command that tests this is:
cargo +nightly rustdoc --config 'build.rustdocflags=["--cfg", "docsrs", "--cap-lints", "warn"]'

I took this from the docs.rs build logs. By running locally, I can confirm that this patch fixes the docs build.

[comphead]

lgtm thanks @alamb

[alamb]

A command that tests this is: cargo +nightly rustdoc --config 'build.rustdocflags=["--cfg", "docsrs", "--cap-lints", "warn"]'

I took this from the docs.rs build logs. By running locally, I can confirm that this patch fixes the docs build.

Thank you @leoyvens -- that is quite clever.