Remove opensearch observability plugin from datanode (6.1)

[graylog-internal-actions-access]

Note: This is a backport of #21045 to 6.1.

This plugin is not used anyhow. Let's reduce distribution size, speed up startup of the opensearch and avoid possible problems during plugin init.

/nocl

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.