Remove makefile

[carolynvs]

What does this change

I have moved the last target, setup-dco, over to mage and removed our Makefile.

SetupDCO will only work on shells that support bash (because the git commit hook is written in bash). Our mage target attempts to detect if running in cmd/powershell and warns the user.

What issue does it fix

No more confusing makefile + magefile.go in our repo! 🎉

Notes for the reviewer

N/A

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR

[carolynvs]

/azp run test-porter-release

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