Omit cleaning up the Affinity Assistant if disabled

[jlpettersson]

Changes

This commit introduces a check if the Assistant is
disabled, and only cleanup the Assistant if disabled

Related to #3212
/kind cleanup

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/pipelinerun/affinity_assistant.go	92.0%	90.4%	-1.6

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/pipelinerun/affinity_assistant.go	92.0%	92.3%	0.3

[jlpettersson]

I also added a unit test to check this property.

[pritidesai]

Related to #3212

Instead of checking the error message apierrors.IsNotFound(err), is it possible to list it and check if it exists? No harm in doing it this way either since the error message wouldn't be reported but will be cleaner to check before issuing delete.

Sorry if this is already discussed somewhere else 🙏

[pritidesai]

thanks @jlpettersson
/lgtm