Revert "New Repository#insights_config method"

This reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26183.

What does this MR do?

Since we changed the Insights config path to be '.gitlab/insights.yml', the Repository#insights_config wouldn't return the config file since Repository#file_on_head can only return files that are at the root of the repo.

Let's revert this MR and I'll use another way of fetching the file with Repository#blob_data_at instead.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer