http2: fix session[kSession] undefined issue

`finishSessionDestroy` session cleanup when already done.

PR-URL: https://github.com/nodejs/node/pull/24547
Fixes: https://github.com/nodejs/node/issues/24546


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>