Replace and remove EE ObjectHierarchy

There's an EE specific version of ObjectHierarchy which only includes a roots method. There's no need for this to be EE specific. We also have new ways to perform the query.

https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/ee/gitlab/object_hierarchy.rb#L7

# frozen_string_literal: true

module EE
  module Gitlab
    module ObjectHierarchy
      # rubocop: disable CodeReuse/ActiveRecord
      def roots
        base_and_ancestors.where(namespaces: { parent_id: nil })
      end
      # rubocop: enable CodeReuse/ActiveRecord
    end
  end
end

We should: