Drop unique index on ci_job_artifacts

Ref: #429002 (closed)

What does this MR do and why?

Following up !138620 (merged) where we introduced a new index including the partition key as you can see on production.

This MR removes the old unique index which does not include the partition key.

- "index_ci_job_artifacts_on_job_id_and_file_type" UNIQUE, btree (job_id, file_type)
+ "idx_ci_job_artifacts_on_job_id_file_type_and_partition_id_uniq" UNIQUE, btree (job_id, file_type, partition_id)

Why are we doing this?

Those indexes are required in order to partition ci_job_artifacts as the primary key and all unique constraints should include the partition key in order to be able to partition our table.

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