Remove unused ci_job_artifact_states_job_artifact_id_seq

Ref: #362984 (closed)

What does this MR do and why?

This MR removes an unused sequence generated by mistake when introducing our ci_job_artifact_states table.

As this is not being used at all by our application, we are setting a default value of 1 when reverting our migration.

Why are we doing this?

We generated those sequences by mistake when setting the primary key by using a different column id.

We missed to set default: nil when creating associated migration as suggested by our documentation.

Setting default: nil will ensure a primary key sequence is not created, and since the primary key will automatically get an index, we set index: false to avoid creating a duplicate.

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