http2: reduce require calls in http2/core

This commit removes unnecesary requires of http and internal/util in
http2/core.js

Backport-PR-URL: https://github.com/nodejs/node/pull/22850
PR-URL: https://github.com/nodejs/node/pull/20422


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>