Remove unused elasticsearch mappings

There are nested mappings for issue in ee/lib/elastic/latest/config.rb that are not used when indexing issue documents. The json generated for issues does not nest these fields inside issue and the fields also exist as top level mappings.

These can be safely removed.

        indexes :issue do
          indexes :assignee_id, type: :integer
          indexes :author_id, type: :integer
          indexes :confidential, type: :boolean
        end