style(rubocop): remaining offences

app/models/concerns/has_authentication.rb

@@ -44,19 +44,4 @@ module HasAuthentication
 
 end
 
-class Postal::Errors::AuthenticationError < Postal::Error
-
-  attr_reader :error
-
-  def initialize(error)
-    super()
-    @error = error
-  end
-
-  def to_s
-    "Authentication Failed: #{@error}"
-  end
-
-end
-
 # -*- SkipSchemaAnnotations

app/models/domain.rb

@@ -85,7 +85,7 @@ class Domain < ApplicationRecord
   def parent_domains
     parts = name.split(".")
     parts[0, parts.size - 1].each_with_index.map do |_, i|
-      parts[i..-1].join(".")
+      parts[i..].join(".")
     end
   end