MINOR: [Java] Remove superfluous add-source execution

[laurentgo]

Rationale for this change

protobuf plugin already adds generated protobuf classes to the list of source directories, so remove the superfluous build-helper:add-source execution

Are these changes tested?

Yes via CI/CD

Are there any user-facing changes?

No

[vibhatha]

@github-actions crossbow submit -g java

[github-actions]

Revision: f223537

Submitted crossbow builds: ursacomputing/crossbow @ actions-d29bec0db0

Task	Status
java-jars
test-conda-python-3.10-spark-v3.5.0
test-conda-python-3.11-spark-master
test-conda-python-3.8-spark-v3.5.0
verify-rc-source-java-linux-almalinux-8-amd64
verify-rc-source-java-linux-conda-latest-amd64
verify-rc-source-java-linux-ubuntu-20.04-amd64
verify-rc-source-java-linux-ubuntu-22.04-amd64
verify-rc-source-java-macos-amd64

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 797ca30.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.

[vibhatha]

@github-actions crossbow submit -g java-jars

[github-actions]

Invalid group(s) {'java-jars'}. Must be one of {'verify-rc-source-linux', 'linux-amd64', 'nightly-tests', 'verify-rc', 'linux', 'verify-rc-source', 'go', 'homebrew', 'verify-rc-binaries', 'nightly', 'verify-rc-wheels', 'conan', 'example-python', 'ruby', 'example-cpp', 'java', 'conda', 'nightly-packaging', 'verify-rc-jars', 'fuzz', 'r', 'vcpkg', 'verify-rc-source-macos', 'linux-arm64', 'python', 'test', 'c-glib', 'integration', 'example', 'wheel', 'cpp', 'nightly-release', 'packaging'}
The Archery job run can be found at: https://github.com/apache/arrow/actions/runs/10085984066

[vibhatha]

@github-actions crossbow submit -g java java-jars

[github-actions]

Revision: f223537

Submitted crossbow builds: ursacomputing/crossbow @ actions-ba68e21443

Task	Status
java-jars
test-conda-python-3.10-spark-v3.5.0
test-conda-python-3.11-spark-master
test-conda-python-3.8-spark-v3.5.0
verify-rc-source-java-linux-almalinux-8-amd64
verify-rc-source-java-linux-conda-latest-amd64
verify-rc-source-java-linux-ubuntu-20.04-amd64
verify-rc-source-java-linux-ubuntu-22.04-amd64
verify-rc-source-java-macos-amd64