ARROW-11881: [Rust][DataFusion] Fix clippy lint

[alamb]

ARROW-11881: [Rust][DataFusion] Fix clippy lint

A linter error has appeared on master somehow:

error: unnecessary parentheses around `for` iterator expression
   --> datafusion/src/physical_plan/merge.rs:124:31
    |
124 |                 for part_i in (0..input_partitions) {
    |                               ^^^^^^^^^^^^^^^^^^^^^ help: remove these parentheses
    |
    = note: `-D unused-parens` implied by `-D warnings`

Seen on at least #9612 and #9639:

https://github.com/apache/arrow/pull/9612/checks?check_run_id=2042047472

https://github.com/apache/arrow/pull/9639/checks?check_run_id=2042649120

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11881

[codecov-io]

Codecov Report

Merging #9642 (c9bb438) into master (35daca2) will increase coverage by 0.06%.
The diff coverage is 88.18%.

@@            Coverage Diff             @@
##           master    #9642      +/-   ##
==========================================
+ Coverage   82.32%   82.39%   +0.06%     
==========================================
  Files         245      245              
  Lines       56412    57170     +758     
==========================================
+ Hits        46442    47105     +663     
- Misses       9970    10065      +95     

Impacted Files	Coverage Δ
rust/benchmarks/src/bin/tpch.rs	38.33% <ø> (ø)
...datafusion/src/physical_plan/crypto_expressions.rs	52.45% <ø> (ø)
rust/datafusion/src/physical_plan/mod.rs	88.00% <ø> (ø)
rust/arrow/src/datatypes/field.rs	55.47% <47.61%> (-0.66%)	⬇️
rust/datafusion/src/physical_plan/parquet.rs	87.83% <57.14%> (-0.22%)	⬇️
...datafusion/src/physical_plan/string_expressions.rs	73.38% <74.59%> (-3.62%)	⬇️
rust/datafusion/src/physical_plan/hash_join.rs	84.16% <77.77%> (+0.64%)	⬆️
rust/arrow/src/datatypes/schema.rs	73.39% <78.57%> (+0.76%)	⬆️
rust/datafusion/src/physical_plan/repartition.rs	81.21% <87.50%> (-0.14%)	⬇️
rust/datafusion/src/physical_plan/functions.rs	89.22% <96.19%> (+3.70%)	⬆️
... and 13 more

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[Dandandan]

LGTM

[alamb]

Thanks @nevi-me and @Dandandan