http2: simplify custom promisify

[BridgeAR]

This is just a small refactoring.

Refs: #20830 (comment)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

@BridgeAR build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/285/pipeline

[addaleax]

I think this changes semantics -- previously, it would resolve the Promise once the connection is established, now it does so immediately.
I think the former is the expected behaviour here.

[BridgeAR]

@addaleax true. Even though there is another way of doing it that would still be more straight forward, I do not really think that it has to be changed. I just tried to follow up on the request. Closing therefore.