Remove redundant ci_builds (status) index

What does this MR do?

Removes redundant index ci_builds on status which is already covered by much wider index.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the database guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/46499