Remove issues.assignee_id column

Open Valery Sizov requested to merge remove_assignee_id into master

What does this MR do?

We migrated to assignee data to a separate table issue_assignees. This field was untouched to be able to rollback safely.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/30453

Edited by Valery Sizov