Revert "change api http.client to interface"

[bwplotka]

Hi,

Thanks @ArthurSens and @tips for doing the work! However, I think the #1387 change might not be necessary. I explained in #1387 (review) --let me know if this makes sense. I think we could update the docs to make it more clear.

TL;DR One can achieve the use case mentioned by @tips in many ways currently

api.Confg{Config: retryablehttp.StandardClient(}}

or

api.Confg{Config: &http.Client{Transport: <Your method with (*Request) (*Response, error) semantics>}}

Let's avoid additional interfaces which looks 99.999% like standard library ones and avoid allowing multiple ways of doing the same thing.