http2: remove unnecessary v8 qualified names

This commit removes v8 qualified identifiers that have using directives
to be consistent with the rest of the code in node_http2.cc.

PR-URL: https://github.com/nodejs/node/pull/20420


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>