Postmigration setenv.sh && common.sh cleanup

[oakrizan]

Proposed commit message

After migration setenv.sh is not needed anymore, since all env vars are defined in related scripts or pipelines.
Added GOX_FLAGS for all steps, as it was in Jenkins.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates: https://github.com/elastic/ingest-dev/issues/3219

Logs

BK builds: https://buildkite.com/elastic/beats/builds?branch=oakrizan%3Apostmigration-setenv-cleanup

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b postmigration-setenv-cleanup upstream/postmigration-setenv-cleanup
git merge upstream/main
git push upstream postmigration-setenv-cleanup

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b postmigration-setenv-cleanup upstream/postmigration-setenv-cleanup
git merge upstream/main
git push upstream postmigration-setenv-cleanup

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b postmigration-setenv-cleanup upstream/postmigration-setenv-cleanup
git merge upstream/main
git push upstream postmigration-setenv-cleanup

[oakrizan]

/test

[oakrizan]

/test

[dliappis]

Great to see so much old cruft getting removed!

LGTM provided that CI is green. Left a minor suggestion but it doesn't require another review cycle.