http2: make --expose-http2 flag a non-op

Make the `http2` module always available.
The `--expose-http2` cli flag is made a non-op

PR-URL: https://github.com/nodejs/node/pull/15535


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>