Move payment method display logic to helper

So we don't need to convert strings into classes to then only convert it
into the same string again.

app/controllers/spree/admin/payment_methods_controller.rb

@@ -138,7 +138,7 @@ module Spree
 
         providers.delete("Spree::Gateway::StripeSCA") unless show_stripe?
 
-        providers.map(&:constantize)
+        providers
       end
 
       # Show Stripe as an option if enabled, or if the