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GH-44223: [Dev] Use "Gandiva" instead of "C++ - Gandiva" label (#44722)

### Rationale for this change

We have Gandiva related codes. So "C++ -" isn't match the current code base.

### What changes are included in this PR?

Remove the "C++ - " part.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* GitHub Issue: #44223

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>