remove-async-handling

[SylvainJuge]

Motivation/summary

• async handling has been removed in apm-data with feat: remove async handling code apm-data#197 and thus needs to be removed in apm-server

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

[mergify]

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

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[marclop]

Verified on ESS using the latest 8.13 BC. Turned on stack monitoring and observed the request handling duration. Using async=true no longer reduces process time close to zero. Additionally, I 5x the heavy.ndjson file contents so that it contained way more events than currently available semaphore.

The expected result would've been a queue is full message. However, because the async query parameter is ignored, it's accepted and processed synchronously.