Refactoring digital shipping calculation and display

* removed the deface override that modified the checkout/_delivery partial
* created custom digital delivery calculator
* removed digital_shipping_method from Spree::Order. Moved logic to available_shipping_methods monkey patch

The main goal was to eliminate the shipping method detection logic from the _delivery partial.
This was a bit of a hack all along and didn't really present the correct information to the underlying
order logic (i.e. rate_hash.blank? would return true when there was no shipping options available).
This should be more future proof and make overriding another aspect of the delivery process easier.

app/models/spree/calculator/digital_delivery.rb

@@ -0,0 +1,18 @@
+# https://github.com/spree/spree/issues/1439
+require_dependency 'spree/calculator'
+
+module Spree
+  class Calculator::DigitalDelivery < Calculator::FlatRate
+    def self.description
+      I18n.t(:digital_delivery)
+    end
+
+    def compute(object=nil)
+      self.preferred_amount
+    end
+
+    def available?(order)
+      order.digital?
+    end
+  end
+end