http2: remove callback-based padding

This option is not useful in practice, as mentioned in comments and the
documentation, because the overhead of calling into JS makes it
unreasonably expensive.

PR-URL: https://github.com/nodejs/node/pull/29144


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>