Remove clear_lti_session_data as it is no longer needed

* no lti session data anymore included in the function
* decided not to delete the pg_id here as it is handled in create_through_lti and implement
* the function is only called once without an exercise id so the values are now directly deleted there

spec/concerns/lti_spec.rb

@@ -17,16 +17,6 @@ describe Lti do
     end
   end
 
-  describe '#clear_lti_session_data' do
-    it 'clears the session' do
-      expect(controller.session).to receive(:delete).with(:external_user_id)
-      expect(controller.session).to receive(:delete).with(:study_group_id)
-      expect(controller.session).to receive(:delete).with(:embed_options)
-      expect(controller.session).to receive(:delete).with(:pg_id)
-      controller.send(:clear_lti_session_data)
-    end
-  end
-
   describe '#external_user_name' do
     let(:first_name) { 'Jane' }
     let(:last_name) { 'Doe' }