Remove redundant email related hieradata from the backend class
Remove redundant email related hieradata from the backend class

This dates back to when the email related services ran on the backend
machines. This is no longer used, as the classes are no longer applied
to these machines.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet