Remove deprecated reference to Spree::MailMethod

Said class is removed in https://github.com/spree/spree/pull/2643 in
Spree.

app/mailers/spree/base_mailer_decorator.rb

@@ -8,8 +8,7 @@ Spree::BaseMailer.class_eval do
   protected
 
   def from_address
-    Spree::MailMethod.current.andand.preferred_mails_from ||
-      'test@example.com'
+    Spree::Config[:mails_from] || 'test@example.com'
   end
 
   def roadie_options

app/models/spree/app_configuration_decorator.rb

@@ -4,6 +4,8 @@ Spree::AppConfiguration.class_eval do
   # we can allow to be modified in the UI by adding appropriate form
   # elements to existing or new configuration pages.
 
+  preference :mails_from, :string, default: 'no-reply@example.com'
+
   # Embedded Shopfronts
   preference :enable_embedded_shopfronts, :boolean, default: false
   preference :embedded_shopfronts_whitelist, :text, default: nil