internal/pkg/agent/cmd: remove redundant addCommandIfNotNil func

[efd6]

What does this PR do?

None of the subcommand construction functions return a nil *cobra.Command, althought this was apparently the case in the past. So remove the wrapper func and its tests.

Why is it important?

Reduce complexity and bugspace.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[mergify]

This pull request does not have a backport label. Could you fix it @efd6? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[efd6]

Not sure what backport policy you want with this.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 7224c77

cc @efd6

[pierrehilbert]

I think we can backport this to 8.x but would like @ebeahan's opinion here (or someone from his team).

[swiatekm]

Looks like this was added in #4908 because the otel command didn't work on Windows and the function would return nil, and then removed in #5767. The change looks good to me and we can backport it everywhere, though for this kind of cleanup, main and 8.x are probably the most appropriate.