Remove unnecessary meta programming from polymorphic_routes.rb

Hi guys,

Just sending out a review request for whomever feels like reviewing:
https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4657-remove-unnecessary-meta-programming-from-polymorphic_routesrb#ticket-4657-2

Very minor patch — just some cleanup in the name of maintainability.

-Jury