remove useless conditionals

`element` can never be a hash because:

1. `slice` returns a Parameters object and calls each on it: https://github.com/rails/rails/blob/cb3f25593b1137e344086364d4b1a52c08e8eb3b/actionpack/lib/action_controller/metal/strong_parameters.rb#L656
2. `each` which is implemented by `each_pair` will call `convert_hashes_to_parameters` on the value: https://github.com/rails/rails/blob/cb3f25593b1137e344086364d4b1a52c08e8eb3b/actionpack/lib/action_controller/metal/strong_parameters.rb#L192-197
3. `convert_hashes_to_parameters` will convert any hash objects in to parameters objects: https://github.com/rails/rails/blob/cb3f25593b1137e344086364d4b1a52c08e8eb3b/actionpack/lib/action_controller/metal/strong_parameters.rb#L550-566

Author: tenderlove

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -578,7 +578,7 @@ def each_element(object)
       end
 
       def fields_for_style?(object)
-        (object.is_a?(Hash) || object.is_a?(Parameters)) &&
+        object.is_a?(Parameters) &&
           object.to_unsafe_h.all? { |k, v| k =~ /\A-?\d+\z/ && v.is_a?(Hash) }
       end
 
@@ -665,7 +665,7 @@ def hash_filter(params, filter)
           else
             # Declaration { user: :name } or { user: [:name, :age, { address: ... }] }.
             params[key] = each_element(value) do |element|
-              if element.is_a?(Hash) || element.is_a?(Parameters)
+              if element.is_a?(Parameters)
                 element = self.class.new(element) unless element.respond_to?(:permit)
                 element.permit(*Array.wrap(filter[key]))
               end