fix(bench): avoid OOM in array_replace bench

[kumarUjjawal]

Which issue does this PR close?

• Closes array_expression benchmark uses too much memory and is sigkill'd by the OS #18447.

Rationale for this change

The array_replace benchmark allocated ~90GB of memory before running (3 × 100M Expr literals), causing OOM on normal machines.

What changes are included in this PR?

• Reduce array_len in array_expression bench from 100_000_000 to 100_000.
• Remove a broken assert_eq! (and its unused expected_array) that compared the ScalarFunction Expr returned by array_replace_all against the unmodified input — the two Expr trees are never equal. The OOM previously hid the failing assertion.

Are these changes tested?

Yes, cargo bench -p datafusion-functions-nested --bench array_expression passes locally

Are there any user-facing changes?

No

[alamb]

Thanks @kumarUjjawal