[JENKINS-68126] Remove event streaming causing performance problems at scale

[Vlatombe]

Amends #1167

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[jglick]

Seems to involve a loss of some diagnosability, but that seems an acceptance price to pay for the scalability improvement (and code simplification).

[jglick]

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[MattLud]

Seems to involve a loss of some diagnosability, but that seems an acceptance price to pay for the scalability improvement (and code simplification).

This removal puts us in a bind for providing user feedback on builds.

Previously, this would inform them that their pod wasn't scheduled due to lack of resources or other capacity issues.

Now we're back to a "spinning beachball" setup - a user needs to ping platform owners to investigate why a pod isn't running.

[jglick]

It would be great to readd support for streaming agent pod event messages (and also pod logs) to the build log, if this can be done without burdening either the controller or the API server.