[main](backport #41489) Revert system module support for journald

[mergify]

After discussing with @cmacknz we understood it would be better to revert this PR to not lock in users with the system-logs "proxy input" at the moment.

Proposed commit message

Revert the system module usage of the system-logs input that was enabling it to run the Journald input. The revert is done in the system module configuration, pipelines and documentation. The system-logs input and its tests are kept.

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.

Disruptive User Impact

## Author's Checklist

How to test this PR locally

Run the tests

mage buildSystemTestBinary
mage docker:ComposeUp
source $(mage PythonVirtualEnv)/bin/activate
INTEGRATION_TESTS=1 BEAT_STRICT_PERMS=false ES_PASS=testing ES_USER=admin TESTING_FILEBEAT_MODULES=system pytest tests/system/test_modules.py

Run the system module

Package filebeat from this PR

# edit modules.d/system.yml and enable both filesets
# edit filebeat.yml and add the ES output and Kibana URL/credentials
./filebeat setup -e -v
./filebeat -e -v

Related issues

• Reverts Use journald for system module on Debian 12 #41061, however it keeps the systemlogs-input implementation and tests.

## Use cases
## Screenshots
## Logs

---

This is an automatic backport of pull request #41489 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 00d7161 has failed:

On branch mergify/bp/main/pr-41489
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit 00d7161a1d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	deleted:    filebeat/docs/include/use-journald.asciidoc
	modified:   filebeat/docs/modules/system.asciidoc
	modified:   filebeat/filebeat.reference.yml
	deleted:    filebeat/module/system/README.md
	modified:   filebeat/module/system/_meta/config.reference.yml
	modified:   filebeat/module/system/_meta/config.yml
	modified:   filebeat/module/system/_meta/docs.asciidoc
	deleted:    filebeat/module/system/auth/ingest/entrypoint.yml
	deleted:    filebeat/module/system/auth/ingest/files.yml
	deleted:    filebeat/module/system/auth/ingest/grok-auth-messages.yml
	deleted:    filebeat/module/system/auth/ingest/journald.yml
	renamed:    filebeat/module/system/auth/ingest/common.yml -> filebeat/module/system/auth/ingest/pipeline.yml
	modified:   filebeat/module/system/auth/manifest.yml
	deleted:    filebeat/module/system/auth/test/debian-12.export
	deleted:    filebeat/module/system/auth/test/debian-12.journal
	deleted:    filebeat/module/system/auth/test/debian-12.journal-expected.json
	deleted:    filebeat/module/system/syslog/ingest/entrypoint.yml
	deleted:    filebeat/module/system/syslog/ingest/journald.yml
	renamed:    filebeat/module/system/syslog/ingest/files.yml -> filebeat/module/system/syslog/ingest/pipeline.yml
	modified:   filebeat/module/system/syslog/manifest.yml
	deleted:    filebeat/module/system/syslog/test/debian-12.export
	deleted:    filebeat/module/system/syslog/test/debian-12.journal
	deleted:    filebeat/module/system/syslog/test/debian-12.journal-expected.json
	modified:   filebeat/modules.d/system.yml.disabled
	modified:   x-pack/filebeat/filebeat.reference.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   filebeat/module/system/auth/config/auth.yml
	both modified:   filebeat/module/system/syslog/config/syslog.yml
	both modified:   filebeat/tests/integration/systemlogs_linux_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[cmacknz]

Will approve once the conflicts are resolved.