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GH-41529: [C++][Compute] Remove redundant logic for ArrayData as ExecResults in ExecScalarCaseWhen (#41380)



### Rationale for this change
Remove useless path in `ExecScalarCaseWhen`

### What changes are included in this PR?
Refactor : remove processing logic for ArrayData as ExecResults in ExecScalarCaseWhen.

### Are these changes tested?
Yes, by exists.

### Are there any user-facing changes?
No

* GitHub Issue: #41529

Authored-by: ZhangHuiGui <2689496754@qq.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>