http2: remove side effects from validateSettings

The function did not only validate the input so far but it also made
a copy of the input object and returned that copy to the callee
function. That copy was not necessary for all call sites and it was
not obvious that the function did not only validate the input but
that it also returned a copy of it. This makes sure the function does
nothing more than validation and copying is happening in the callee
function when required.

PR-URL: https://github.com/nodejs/node/pull/26809


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>