Cleanup publish mage target

[carolynvs]

In a follow-up, let's remove the argument and switch this repo over to use mage 100% of the time, removing the makefile. You can look at the kubernetes plugin magefile as an example.

Long term it would be great to have an equivalent plugins/magefile.go in the magefiles repository just like we have with mixins, so that the entire magefile is easy to manage without requiring a lot of customization.

Originally posted by @carolynvs in #41 (comment)

[kichristensen]

Seems to have been done in #45