Remove Mapper class

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

This PR removes the Mapper helper class, as well as lots of now-unused code in Requestor.

This is technically a breaking API change, but most users shouldn't be impacted:

• Mapper was public, so some users might be using Mapper<StripeEntityType>.MapFromJson() to deserialize Stripe objects. Users can still deserialize Stripe objects by calling StripeEntityType.FromJson(), or directly calling Newtonsoft.Json.JsonConvert.DeserializeObject<StripeEntityType>()
• StripeResponse.ObjectJson is removed. I don't think it was particularly useful. (The full JSON is still available in ResponseJson.)

[remi-stripe]

I'm sure people are using Mapper in some ways so I'd still explicitly call this a breaking change (but no need to mark as obsolete first)

[ob-stripe]

I'm sure people are using Mapper in some ways so I'd still explicitly call this a breaking change (but no need to mark as obsolete first)

Yup, will do. Thanks @remi-stripe!