Remove unnecessary parentheses accidentally reintroduced in #1017.

Author: beauby

lib/active_model/serializer/adapter/json_api.rb

@@ -43,7 +43,7 @@ def serializable_hash(options = nil)
 
         def fragment_cache(cached_hash, non_cached_hash)
           root = false if @options.include?(:include)
-          ActiveModel::Serializer::Adapter::JsonApi::FragmentCache.new().fragment_cache(root, cached_hash, non_cached_hash)
+          ActiveModel::Serializer::Adapter::JsonApi::FragmentCache.new.fragment_cache(root, cached_hash, non_cached_hash)
         end
 
         private