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ARROW-11034: [Rust] remove rustfmt ignore list, fix format

In this PR:

- Ignore list is commented out from rustfmt.toml, thus `carrgo +stable fmt` would not show warnings
- Two files are re-formatted by nightly, stable: this may avoid formatting problem we had seen in CI (hope so).

Shell commands that I had run:
```
cargo +nightly-2020-11-24-x86_64 fmt
cargo +1.48.0-x86_64 fmt
```

Closes #9013 from mqy/ARROW-11034_rustfmt

Authored-by: mqy <meng.qingyou@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>