Don't set processor.event for logs

[axw]

Motivation/summary

This is not used in any queried made by the APM UI, and is an APM-specific field that we should phase out. By removing it from logs, we can potentially write logs to non-APM specific log datastreams.

See also elastic/kibana#161718

Checklist

- [ ] Update CHANGELOG.asciidoc

• Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

1. Send some traces and logs, e.g. using https://github.com/elastic/apm-server/tree/main/systemtest/cmd/sendotlp
2. Check that the logs data stream does not have a processor.event field
3. Check that the logs still show up in the APM UI

Related issues

elastic/kibana#161718

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.