Remove Duplicate Error Message When Deleting Nonexistent Resource

[danielhelfand]

Closes #814

This pull request removes the printing of the error message in deleter.go, which causes a duplicate error message to be printed in the case of deleting a resource that doesn't exist. It also moves testing of this case to delete subcommands for each resource.

Submitter Checklist

• Includes tests (if functionality changed/added)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

Release Notes

Remove duplicate error message when deleting nonexistent resource

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-cli-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/deleter/deleter.go	100.0%	76.9%	-23.1

[pradeepitm12]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: piyush-garg, pradeepitm12

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [piyush-garg]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment