add migration

db/post_migrate/20180202111106_remove_project_labels_group_id.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveProjectLabelsGroupId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    update_column_in_batches(:labels, :group_id, nil) do |table, query|
+      query.where(table[:type].eq('ProjectLabel').and(table[:group_id].not_eq(nil)))
+    end
+  end
+
+  def down
+  end
+end