Simplify set_week callbacks

Changes to :week are already ignored py paper_trail. No need to wrap this in a
`without_versioning`. Also drop the reader in `Registration` and the tests. Why
write this into the DB but never read it again? Why test standard
ruby/activerecord stuff.

app/models/event.rb

@@ -317,10 +317,7 @@ class Event < ApplicationRecord
   end
 
   def set_week
-    self.week = created_at.strftime('%W')
-    paper_trail.without_versioning do
-      save!
-    end
+    update!(week: created_at.strftime('%W'))
   end
 
   def before_end_of_conference

app/models/registration.rb

@@ -46,10 +46,6 @@ class Registration < ApplicationRecord
     (conference.program.events.with_registration_open - events) + events
   end
 
-  def week
-    created_at.strftime('%W').to_i
-  end
-
   private
 
   ##
@@ -76,10 +72,7 @@ class Registration < ApplicationRecord
   end
 
   def set_week
-    self.week = created_at.strftime('%W')
-    paper_trail.without_versioning do
-      save!
-    end
+    update!(week: created_at.strftime('%W'))
   end
 
   def registration_limit_not_exceed

spec/models/registration_spec.rb

@@ -40,16 +40,6 @@ describe 'Registration' do
   describe 'after create' do
     after { subject.run_callbacks(:create) }
 
-    # set_week and subscribe_to_conference are private methods
-    describe '#set_week' do
-      before { subject.created_at = Time.utc(2014, 5, 10) }
-
-      it 'sets week of registration' do
-        expect(subject).to receive(:set_week)
-        expect(subject.week).to eq 18
-      end
-    end
-
     describe '#subscribe_to_conference' do
       it 'subscribes to conference' do
         expect(subject).to receive(:subscribe_to_conference)
@@ -62,14 +52,6 @@ describe 'Registration' do
     end
   end
 
-  describe '#week' do
-    before { subject.created_at = Date.new(2014, 06, 30) }
-
-    it 'returns week number of created_at' do
-      expect(subject.week).to eq(26)
-    end
-  end
-
   describe 'registration_to_events_only_if_present' do
     context 'valid' do
       it 'when user registers for events happening while user is at the conference' do