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GH-40540: [CI][C++] Don't install FlatBuffers (#40541)

### Rationale for this change

We don't need it because we're using bundled one.

### What changes are included in this PR?

Remove FlatBuffers install commands.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #40540

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>