bug: Eliminate dead round-robin insertion in enforce distribution (#19132)

## Which issue does this PR close?

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- Closes #19131.

## Rationale for this change

Eliminates dead code that was not used in the enforce distribution rule.

In the branch where a parent requires hash repartitioning, there was a
condition that would "add" a round robin if hash was not necessary
(already hashed correctly) and the add round-robin flag was marked as
true. This condition would never evaluate to true because anytime a
parent requires hash repartitioning, we cannot round-robin because it
would break the hash partitioning.
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## What changes are included in this PR?

A condition is deleted. No tests or plans were changed since this was
dead code.
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## Are these changes tested?

Yes, all tests (unit and sqllogictests) still pass.

There is no new tests to add.
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## Are there any user-facing changes?

No.
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Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: gene-bordegaray

datafusion/physical-optimizer/src/enforce_distribution.rs

@@ -1274,11 +1274,6 @@ pub fn ensure_distribution(
                 }
                 Distribution::HashPartitioned(exprs) => {
                     // See https://github.com/apache/datafusion/issues/18341#issuecomment-3503238325 for background
-                    if add_roundrobin && !hash_necessary {
-                        // Add round-robin repartitioning on top of the operator
-                        // to increase parallelism.
-                        child = add_roundrobin_on_top(child, target_partitions)?;
-                    }
                     // When inserting hash is necessary to satisfy hash requirement, insert hash repartition.
                     if hash_necessary {
                         child =