Cleanup logic for API convertion errors

[afrittoli]

Changes

When we used to store v1alpha1, the get pipeline and task data
function could return an error related to coverting between
API versions. Now that we store v1beta1, that error is not
returned anymore by those functions, so we can cleanup the
error handling specific to it.

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/kind cleanup

[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/pipelinerun.go	82.0%	83.0%	1.1
pkg/reconciler/taskrun/taskrun.go	77.4%	77.6%	0.2

[vdemeester]

/lgtm

[tekton-robot]

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[tekton-robot]

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