Drop log_id foreign key from user migrations
user
```ruby
Sequel.migration do
  up do
    alter_table :user_migration_exports do
      drop_constraint :user_migration_exports_log_id_fkey
    end 
    alter_table :user_migration_imports do
      drop_constraint :user_migration_imports_log_id_fkey
    end 
  end 

  down do
    alter_table :user_migration_exports do
      add_foreign_key [:log_id], :logs, null: false
    end 
    alter_table :user_migration_imports do
      add_foreign_key [:log_id], :logs, null: false
    end 
  end 
end

```
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