[8.19](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

  ---

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

[mergify]

Cherry-pick of f4b9ad1 has failed:

On branch mergify/bp/8.19/pr-47804
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit f4b9ad1dd.
  (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:   dev-tools/mage/build.go
	modified:   dev-tools/mage/gotest.go
	deleted:    libbeat/otelbeat/providers/fbprovider/fbprovider.go
	deleted:    libbeat/otelbeat/providers/fbprovider/fbprovider_test.go
	deleted:    libbeat/otelbeat/providers/mbprovider/mbprovider.go
	deleted:    libbeat/otelbeat/providers/mbprovider/mbprovider_test.go
	deleted:    libbeat/otelbeat/providers/providers.go
	deleted:    x-pack/filebeat/cmd/otelcmd_disabled.go
	deleted:    x-pack/filebeat/cmd/otelcmd_enabled.go
	modified:   x-pack/filebeat/cmd/root.go
	modified:   x-pack/filebeat/magefile.go
	deleted:    x-pack/metricbeat/cmd/otelcmd_disabled.go
	deleted:    x-pack/metricbeat/cmd/otelcmd_enabled.go
	modified:   x-pack/metricbeat/cmd/root.go
	modified:   x-pack/metricbeat/magefile.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: metricbeat/module/kubernetes/util/prometheus.go
	deleted by them: x-pack/libbeat/common/otelbeat/otel.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

[elasticmachine]

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