Stop displaying X-CF-Warning headers to the user.

- They're not actionable by the user.
- They're scary to the user.
- They're only used for deprecated endpoints at the moment.

- In order to get the benefit of knowing when we're using deprecated
  endpoints, we added behavior to the CLI where it will panic with
  errors if the CF_RAISE_ERROR_ON_WARNINGS environment variable is set.
  This means that we can have our acceptance tests run with that
  environment variable set, which will cause our tests to explode when
  we use a deprecated endpoint, which will cause us to notice that we're
  using a deprecated endpoint, which will induce us to fix it ASAP.

  It's a much better system than sitting and waiting for user complaints
  about scary deprecation messages from "push".

  [Finishes #76833522]