Remove redundant NullSerializer

Use one from ActiveSupport::MessageEncryptor module.

/cc @tamird @chancancode
No, it's sufficient. I believe that it is also misleading, because it implies to a reader that these implementations may not be identical. I can't imagine a scenario in which having two implementations of `NullSomethingDoer` is reasonable, so I argue that we should not encourage it by aliasing the constant.