Minor normative change to RFC6265bis

Hello http-wg,

I recently landed a minor change to RFC6265bis which removes a reference to
RFC3490 and the recommendations for how and when to use it.
https://github.com/httpwg/http-extensions/commit/121e2c91d8e55c48400b0e689e00c07cf25eb69e

I want to give the working group a chance to review this change since
RFC6265bis already went through WGLC.

If there aren't any concerns by the end of Jan 6th I'll submit the latest
draft to the chairs.

Thanks all,
- Steven