Cherry-pick #21616 to 7.x: Prevent reporting ecs version twice

[michalpristas]

Cherry-pick of PR #21616 to 7.x branch. Original message:

What does this PR do?

Fixes: #20666

Due to this, we get two ecs.versions in a resulting event.

e.g

...

        "ecs": {
            "version": "1.6.0"
        },
        "ecs.version": "1.5.0",
        "event": {
            "dataset": "elastic.agent.metricbeat"
        },
...

Funny thing is also that it might differ in version as in the example above. But this may be just due to different builds of agent and beat locally.

Why is it important?

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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #21969 opened]

• Start Time: 2020-10-19T12:16:08.769+0000

• Duration: 37 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1394
Skipped	4
Total	1398

[blakerouse]

Backport looks good.