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GH-43378: [Java][CI] Don't configure multithreading when building javadocs (#43674)

### Rationale for this change

Apparently some maven plugins are not thread safe and started throwing errors in the `test-debian-12-docs` CI job when building javadocs.

### What changes are included in this PR?

* Remove multithreading config when building javadocs

### Are these changes tested?

CI

### Are there any user-facing changes?

No
* GitHub Issue: #43378

Authored-by: Dane Pitkin <dpitkin@apache.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>