Add deadcode/unused linters to repositories

[aaronshurley]

Why

With the addition of linting tools, we should continue adding on additional linters to assure better code quality. Some of the more important linters we can add are deadcode/unused. These linters help detect unused code in go projects.

As part of enabling these linters, we should also remove any unused source code from kapp, imgpkg, kbld, and ytt.

As a Carvel maintainer I want linters enabled that check for unused code So that I can avoid confusion/having to maintain code that is not being used

Acceptance Criteria

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Scenario:

Given I run the linter.sh script for the repos listed above Then I see no violations from (deadcode) or (unused)

Additional Infomation

• The linters can be enabled by adding the deadcode and unused under the enable property of the .golangci.yml file as shown here.

[microwavables]

moved from /carvel-community to /carvel as we are merging the two into one