Forget suscripciones

This PR only removes stuff and **breaks compatibility** as it requires the host app to implement the method `enviar_newsletter_a_suscriptores_tematica`. That way this engine has no knowledge about the suscription mechanism.

**It's not urgent to merge** as [the new suscription mechanism](https://github.com/Soluciones/suscribir) is compatible with the current version of the engine, but I wanted this engine to be more decoupled of the suscription stuff.