Revert "Work around broken 'go get golang.org/x/lint'" to utilize GOPROXY on Go 1.13+

What does this MR do?

Reverts d2c430aa.

Why does this MR do?

As with !69 (merged) Go 1.13, which supports GOPROXY set by default (https://golang.org/doc/go1.13#modules), is now used, we do not have to use github.com as proxy.

What are the relevant issue numbers?

n/a

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added
• [n/a] Documentation created/updated for GitLab EE, if necessary
• [n/a] Documentation created/updated for this project, if necessary
• [n/a] Documentation reviewed by technical writer or follow-up review issue created
• [n/a] Tests added for this feature/bug
• [n/a] Job definition updated, if necessary
  • [n/a] Auto-DevOps template
  • [n/a] Job definition example
  • [n/a] CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• [n/a] Security reports checked/validated by reviewer