Improve invalid HTTP status code responses failure mode handling (#2522)

If the model only uses the `@http` trait to define the response code,
Smithy already checks that this is a valid status code, so the
conversion cannot fail, and we can avoid rejecting with
`ResponseRejection::InvalidHttpStatusCode`.

This commit also removes this variant from RPC protocols, where HTTP
binding traits are not allowed, and so this failure mode can never
happen.