Remove security_reports hash from environment entitity

What does this MR do?

We decided that the merged BE implementation needs to be redone. As the current implementation is not used by any consumer, we should remove it.

For more information see #4299 (closed)

This MR reverts the changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6987 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7228

What are the relevant issue numbers?

#4299 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?