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

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 apache#9612 and apache#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

Closes apache#9642 from alamb/fix_clippy

Authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>

rust/datafusion/src/physical_plan/merge.rs

@@ -121,7 +121,7 @@ impl ExecutionPlan for MergeExec {
 
                 // spawn independent tasks whose resulting streams (of batches)
                 // are sent to the channel for consumption.
-                for part_i in (0..input_partitions) {
+                for part_i in 0..input_partitions {
                     let input = self.input.clone();
                     let mut sender = sender.clone();
                     tokio::spawn(async move {