Remove old fields when transforming api request

If the old 'firstname' and 'lastname' fields exist in the request after the transformation wrapper the request handling fails with "unknown field" errors. The goal of the transformation is to make v2 API requests indistinguishable from v3 API requests, so it makes sense to remove the now invalid keys from the request dict.
Since this is inside the `if` block, you don't need to use `.get` with a default value, you can just do `body_dict['firstname']`