Remove unnecessary guard clause

Previously, payment actions that were listed without an associated
`can_?` method were interpreted as supported. All payment methods are
implementing all `can_?` methods for listed actions though. And I think
that new payment methods should explicitely implement all `can_?`
methods instead of relying on this hidden logic.

app/models/spree/payment.rb

@@ -155,8 +155,7 @@ module Spree
       return [] unless payment_method.respond_to?(:actions)
 
       payment_method.actions.select do |action|
-        !payment_method.respond_to?("can_#{action}?") ||
-          payment_method.__send__("can_#{action}?", self)
+        payment_method.__send__("can_#{action}?", self)
       end
     end