Remove return error in GetTaskFunc and GetPipelineFunc

[Yongxuanzhang]

Changes

This commit removes the error return in GetTaskFunc and GetPipelineFunc, no errors are ever created and returned in these functions and should be removed.

/kind cleanup

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[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	85.6%	86.2%	0.6
pkg/reconciler/taskrun/taskrun.go	81.5%	82.1%	0.5

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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[dibyom]

Nice find @Yongxuanzhang !

/lgtm