[Rust] rustfmt cleanup

[asfimport]

We have basically switched to the stable rust after #9698 and #8930.

There is a annoying problem that:

cargo +stable fmt 

prints warning like this:

 

Warning: can't set `ignore ... unstable features are only available in nightly channel.

Since "arrow/src/ipc/gen" has been formatted with the stable, we can remove the ignore list from rustfmt.toml

Reporter: Qingyou Meng / @mqy
Assignee: Qingyou Meng / @mqy

PRs and other links:

• GitHub Pull Request #9013

_{Note: This issue was originally created as ARROW-11034. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9013
#9013