Athens: Remove -mod=vendor from source and docs

[marpio]

https://github.com/gomods/athens/pull/1231 removed the vendor directory but there are still some -mod=vendor left in our Makefile and docs.

We should remove those and use GOPROXY=https://proxy.golang.org instead to build Athens.

[penthaapatel]

Hi! New contributor here. :) I would like to work on this.

[marpio]

Please do @penthaapatel It's yours!

[marpio]

Let us know if you have any questions.

[penthaapatel]

Should I change all the occurrences that say "GOPROXY=http://127.0.0.1:3000" to "GOPROXY=https://proxy.golang.org" in docs/content?

[arschles]

@penthaapatel not everywhere, no. Many of the places in docs that say GOPROXY=http://127.0.0.1:3000 are instructions for how to configure your machine to use an installed Athens, so don't change that. Only change the occurrences that talk about how to build Athens itself. Lots of them will be in development and testing docs

[penthaapatel]

@arschles Thanks! I got it.

[penthaapatel]

I removed -mod=vendor from Makefile, docs/content/install/build-from-source.md, and docs/content/walkthrough.md. Am I doing this right?

[arschles]

@penthaapatel sounds like it! Would you be able to submit this as a pull request and we can start giving feedback directly on the code? Or, did you already submit one and I missed it?

[penthaapatel]

@arschles Okay, I'll submit a PR now. :)