[Fleet] Remove output conditionnal topic

[nchaulet]

Remove output conditional topic as it no longer supported https://github.com/elastic/ingest-dev/issues/2838

Kafka output now only support one topic.

[github-actions]

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[mergify]

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

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.