fix source RB issue

[hboutemy]

see https://github.com/jvm-repo-rebuild/reproducible-central/blob/master/content/org/jpmml/jpmml-xgboost/README.md
release rrequires a recent maven-source-plugin version
while at it, dropping not necessary timestamps: root one is sufficient

check with mvn artifact:check-buildplan -Psonatype-oss-release

[hboutemy]

@vruusmann this PR was not merged for latest release, source is not yet reproducible (same for statsmodel and h2o, even if I did not yet prepare the PRs waiting for this one to get merged)

[vruusmann]

this PR was not merged for latest release

Yep, I forgot to look into open GitHub issues while preparing for the 1.8.6 release.

source is not yet reproducible

Yep, the binary is reproducible, the source isn't.

.. same for statsmodel and h2o

There is currently no maven-source-plugin plugin specification in the JPMML-Converter project, and in all its "downstream" projects.

(even if I did not yet prepare the PRs waiting for this one to get merged)

No need to PR there - I have it mentioned in my internal TODO list.