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/controllers/sessions_controller_spec.rb

@@ -190,7 +190,10 @@ describe SessionsController do
       end
 
       it 'clears the session' do
-        expect(controller).to receive(:clear_lti_session_data)
+        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)
         delete :destroy
       end
 
@@ -210,11 +213,6 @@ describe SessionsController do
       perform_request.call
     end
 
-    it 'clears the session' do
-      expect(controller).to receive(:clear_lti_session_data)
-      perform_request.call
-    end
-
     expect_http_status(:ok)
     expect_template(:destroy_through_lti)
   end