http2: cleanup of h2 compat layer, add tests

Remove unnecessary variable assignments, remove unreachable code
pathways, remove path getter and setter, and other very minor
cleanup. Only remove reference to stream on nextTick so that
users and libraries can access it in the finish event.

Fixes: https://github.com/nodejs/node/issues/15313
Refs: https://github.com/expressjs/express/pull/3390
PR-URL: https://github.com/nodejs/node/pull/15254


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