Remove check for kwargs in snowplow spec helper with Ruby 2.7

With !44711 (merged) we fixed the **kwargs check for expecting Snowplow events since it would otherwise provide an empty Hash {} as an argument.

This gets fixed with Ruby 2.7 where Double splat with an empty hash (**{}) passes no arguments anymore, see: https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

We therefore can revert the change made in !44711 (merged)