Fix event deletion in the memqueue

[swiatekm]

Proposed commit message

Fix event deletion in the memqueue

#39584 accidentally removed Batch.FreeEntries, which was responsible for actually deleting events from the queue. Since that change, events are never deleted from the queue, just overwritten. As a result, we consume noticably more memory wherever the event rate is significantly smaller than the queue size.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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.

Related issues

• Related to Kubernetes metadata overwhelms memory limits in the Agent process elastic-agent#4729, I believe it's the main culprit there.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @swiatekm? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[swiatekm]

This PR will soon be closed in favor of a superior alternative. Until then, I'm keeping it here for reference.

[swiatekm]

Closing in favor of #41356.