Drop potentially unneeded index `tmp_build_stage_position_index`

We currently have this index:

CREATE INDEX tmp_build_stage_position_index ON ci_builds USING btree (stage_id, stage_idx) WHERE (stage_idx IS NOT NULL);

This goes back to the "init schema" migration, so has been introduced a long time ago. The naming suggests this is a temporary index only - do we still need it?

If not, let's drop it.

GitLab.com: 43 GB total size, 24 GB btree bloat estimate.