[8.x](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

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

[elasticmachine]

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