Restore memory queue's internal event cleanup after a batch is vended

[faec]

Fix #41355, where event data in the memory queue was not being freed when event batches were acknowledged, but only gradually as the queue buffer was overwritten by later events. This gave the same effect as if all beat instances, even low-volume ones, were running with a full / saturated event queue.

The root cause, found by @swiatekm, is this PR, an unrelated cleanup of old code that accidentally included one live call along with the deprecated ones. (There was an old FreeEntries hook in pipeline batches that was only used for deprecated shipper configs, but the cleanup also removed the FreeEntries call inside the queue which was essential for releasing event memory.)

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.

Related issues

• Fixes Queue keeps stale event data in memory in 8.15 #41355.

[elasticmachine]

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

[jlind23]

Dummy question - Any reason why the disk queue freeentries method does not do anything?

[rdner]

All changes make sense to me.