[9.2](backport #47804) chore(otel): remove otelbeat

[mergify]

Proposed commit message

The otelbeat binary is not used anymore since we moved to an in-process testing collector in #47338, so remove the subcommand and all references to it in the build system. The otel subcommand was gated behind a build tag and only used in tests.

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Related issues

• Relates otel: update integration tests to use an in-process testing collector #47338
• Relates otel: fix remaining tests to use in process testing collector #47772
• Closes https://github.com/elastic/ingest-dev/issues/6207

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  This is an automatic backport of pull request chore(otel): remove otelbeat #47804 done by Mergify.

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