Clarify that the diff view setting always comes from the cookie

This invalidates one test, which we've removed.

app/controllers/projects/application_controller.rb

@@ -83,8 +83,7 @@ class Projects::ApplicationController < ApplicationController
   end
 
   def apply_diff_view_cookie!
-    view = params[:view] || cookies[:diff_view]
-    cookies.permanent[:diff_view] = params[:view] = view if view
+    cookies.permanent[:diff_view] = params.delete(:view) if params[:view].present?
   end
 
   def builds_enabled

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -300,14 +300,6 @@ describe Projects::MergeRequestsController do
 
       expect(response.cookies['diff_view']).to eq('parallel')
     end
-
-    it 'assigns :view param based on cookie' do
-      request.cookies['diff_view'] = 'parallel'
-
-      go
-
-      expect(controller.params[:view]).to eq 'parallel'
-    end
   end
 
   describe 'GET commits' do