Remove "How to use", "Environment Variables" sections from README

What does this MR do?

This MR removes redundant documentation which has now been moved to the Container Scanning docs as part of the following MR: gitlab-org/gitlab!24583 (merged)

What are the relevant issue numbers?

This is a small change, no releated issue

Does this MR meet the acceptance criteria?

• Changelog entry added Readme only change, no Changelog necessary
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer