Drop temporary index on ci builds

What does this MR do?

This drops a temporary index we don't need/use anymore. See #295477 (closed) for details.

Between 2020-12-23 and 2021-01-06, the index hasn't been used on GitLab.com (we used select * from pg_stat_user_indexes where indexrelname = 'tmp_build_stage_position_index' to check). For details see the linked issue.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content