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GH-34513: [CI][Python] Remove unused imports from _acero.pyx to fix linting failures (#34514)

### Rationale for this change

PR linting jobs are failing

### What changes are included in this PR?

Lint fixes :)

### Are these changes tested?

On CI

### Are there any user-facing changes?

No
* Closes: #34513

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>