http2: switch to new runtime-controlled debugging system

Remove `--debug-http2` as a compile-time feature and
make all debug statements available using `NODE_DEBUG_NATIVE=http2`
at runtime.

This probably makes the debugging-enabled case a bit slower due to
additional string concatenations, but switching to a runtime-checking
system makes debugging more flexible and can be applied more easily
to other parts of the source code as well.

PR-URL: https://github.com/nodejs/node/pull/20987


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>