Drop issues.correct_work_item_type_id foreign key

What does this MR do and why?

This is causing errors in production: gitlab-com/gl-infra/production#18692 (closed) so we drop this temporarily since this is still unused. This was added in !167510 (merged)

This is a NOT VALID constraint on the issues table but it causes other updates to fail with:

PG::ForeignKeyViolation: ERROR:  insert or update on table "issues" violates foreign key constraint "fk_1adaba52b0"
DETAIL:  Key (correct_work_item_type_id)=(0) is not present in table "work_item_types".

We have a trigger to fill in correct_work_item_type_id but that only gets triggered when work_item_type_id is also part of the updated columns.

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