.circleci: remove go mod checks for go 1.12

[debnil]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

This PR removes the go mod-related checks for Go 1.12.

Why

Go 1.13-specific projects like Hubble creates divergence in the dependencies, and the spread of 1.13 in the larger community makes continued support of 1.12 less important in the longer term. Additionally, community best practices advise against running these checks on multiple Go versions.

Known limitations

N/A

[leighmcculloch]

Looks good. I think we can remove the entire check though.

---

One note on why we're okay making this change from the PR description:

makes continued support of 1.12 less important

Go 1.12 support is still as important as supporting 1.13 as it's a supported version in our list of supported versions in the README, but I think the distinction is that these checks exist to keep our code files clean. The go.list file exists so that we're aware of what versions are in use in our applications when we build them ourselves and ship binaries. Since our binaries are built only with Go 1.13 having it with Go 1.13 values only is sufficient I think. If someone else builds with Go 1.12 from source we're not really able to guarantee consistency, I'm not sure if we need to mention that somewhere. It should be consistent, but technically the Go module resolver could pick a different version or something if something major changed between versions. I don't think it's worth maintaining two separate go.list files for each version. The go.list file is not relevant for the SDK packages where people are importing our code.

[leighmcculloch]

👍