Fix cop RedundantPresenceValidationOnBelongs

- presence: true is redundant since Rails 5.0 BUT applies
  with new default config of
  belongs_to_required_by_default to true
  Lots of files with belongs_to_required_by_default = false
  (backward compatibility)
  So: deleting this setting implies to adding optional: true
  on belongs_to relations where there is no explicit check for presence.
  And also to delete redundant presence: true
  The implicit becomes the explicit and vice versa
- updated toto list

spec/models/subscription_line_item_spec.rb

@@ -5,11 +5,11 @@ require 'spec_helper'
 describe SubscriptionLineItem, model: true do
   describe "validations" do
     it "requires a subscription" do
-      expect(subject).to validate_presence_of :subscription
+      expect(subject).to belong_to :subscription
     end
 
     it "requires a variant" do
-      expect(subject).to validate_presence_of :variant
+      expect(subject).to belong_to :variant
     end
 
     it "requires a integer for quantity" do

spec/models/tag_rule_spec.rb

@@ -5,7 +5,7 @@ require 'spec_helper'
 describe TagRule, type: :model do
   describe "validations" do
     it "requires a enterprise" do
-      expect(subject).to validate_presence_of(:enterprise)
+      expect(subject).to belong_to(:enterprise)
     end
   end
 end