Remove GO111MODULE on updating to Go 1.13

What does this MR do?

Remove GO111MODULE variable on image build while Go was upgraded to 1.13 in !21 (merged)

What are the relevant issue numbers?

Part of gitlab-org/security-products&2 (closed)

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

Part of gitlab-org/security-products&2 (closed)