Setting knative-eventing-injection=disabled should clean up auto-Broker

[mattmoor]

Describe the bug
When a namespace has had a Broker automatically generated, either by opt-in (namespace) or default (mtnamespace), applying the label knative-eventing-injection=disabled should clean up the auto-generated resources.

The actual behavior today is that they just stick around.

[grantr]

👍 for this feature. Is the disabled value actually documented somewhere, or is this a feature request?

[mattmoor]

I'm not sure we document disabled anywhere (yet, I have a PR), though I'd argue that removing the enabled label should revert to the default state, so similar effect for the namespace reconciler. Today cleaning up the injected Broker is a bit tedious (disable injection, and know what to delete).

[grantr]

👎 on removing the label also deleting the Broker. The Broker is stateful, so deleting it could cause data loss. Using the absence of a label as an indicator that the user wants their data discarded seems dangerous to me. Safer to wait for a positive indicator like the disabled value.

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