removes user_id column from User model to use idiomatic Rails automatic IDs

spec/vulnerabilities/sensitive_data_exposure.rb

@@ -13,7 +13,7 @@ feature "sensitive data exposure" do
   scenario "attack\nTutorial: https://github.com/OWASP/railsgoat/wiki/A6-Sensitive-Data-Exposure-Cleartext-Storage-SSNs" do
     login @normal_user
 
-    visit "/users/#{@normal_user.user_id}/work_info"
+    visit "/users/#{@normal_user.id}/work_info"
     pending if verifying_fixed?
     expect(page.source).to include "999-99-9999"
   end