remove pipeline bus v1

[yaauie]

Release notes

• removes deprecated v1 pipeline bus; the v2 implementation has been the default since its introduction in Logstash 8.15.0

What does this PR do?

Removes the v1 implementation of the pipeline bus, which was only accessible through a never-documented system property escape hatch.

The property is deprecated on the 8.x branch in #16643

Why is it important/What is the impact to the user?

It should have no user impact; the escape hatch was never documented, and is not expected to be used in the real world.

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 (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

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• Commit: fd94143

[donoghuc]

Looks like v1 was contained to just that single implementation file. Assuming #16643 deprecation is communicated in 8.x this removal looks clean.