libpod: simplify unnecessary loops

[caxu-rh]

This PR changes a few spots in libpod where loops are currently used for extracting keys from a map into a slice, or for checking the existence of a key in a map. These can simplified to remove the loop. There should be no change in behavior and I doubt that there would be performance differences; the intended improvement here is on readability.

There are other similar changes that could be made, but the changes here are the most 'obvious'/agreeable of them.

Checklist

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• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all
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  the sign-off email address. See CONTRIBUTING.md
  for more information.
• Referenced issues using Fixes: #00000 in commit message (if applicable) (none)
• Tests have been added/updated (or no tests are needed) (no expected behavior changes)
• Documentation has been updated (or no documentation changes are needed) (no expected behavior changes)
• All commits pass make validatepr (format/lint checks) - When running locally, failed to build Darwin podman-machine - for libkrun, build constraints exclude all Go files in ..., pretty sure unrelated
• Release note entered in the section below (or None if no user-facing changes)

Does this PR introduce a user-facing change?

None

[baude]

LGTM

[TomSweeneyRedHat]

LGTM
restarted tests that looked to be flakes