Remove enable_mail_delivery preference from DB

It's no longer used.

db/migrate/20210125123000_remove_enable_mail_delivery_preference.rb

@@ -0,0 +1,8 @@
+class RemoveEnableMailDeliveryPreference < ActiveRecord::Migration
+  def up
+    Spree::Preference.delete_all("key ilike '%enable_mail_delivery%'")
+  end
+
+  def down
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20210115143738) do
+ActiveRecord::Schema.define(version: 20210125123000) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"