Remove unused test builder methods

[imjasonh]

/area testing
/kind cleanup
#3178

Found by grep monstrosity in #3178 (comment)

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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
internal/builder/v1beta1/pipeline.go	81.8%	85.7%	3.9
internal/builder/v1beta1/pod.go	78.0%	95.1%	17.1
internal/builder/v1beta1/step.go	19.0%	42.1%	23.1
internal/builder/v1beta1/task.go	75.4%	81.0%	5.6

[vdemeester]

/lgtm
/meow

[tekton-robot]

@vdemeester:

In response to this:

/lgtm
/meow

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[tekton-robot]

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