http2: remove unnecessary asserts in core

options.getTrailers & options.selectPadding will always be a
function by the time the code reaches the assert checks. And
even if not (which seems currently impossible), the assert
failure will be almost the same as calling something that is
not a function. The downside of leaving these is that typeof
checks are decently expensive.

PR-URL: https://github.com/nodejs/node/pull/16327


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>