Cleanup: Remove unused ACK tracking in awscloudwatch input

[faec]

Remove EventACKTracker from awscloudwatch event handling.

EventACKTracker is buggy (see #38961) but in the awscloudwatch input it's also never used -- its callbacks are attached to events, but their result doesn't affect any of the event handling.

This PR doesn't change any functional behavior.

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.

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[zmoog]

I see the cloudwatch input never waits for the pending acks count to reach zero, so removing it makes sense.

LGTM.