[web] Don't handle promise for onDeviceTokenRegistered call in PushNotifsHandler

Summary:
Separating this out because it might need more detailed review.

There's a question I asked Michal on Linear about whether Electron apps crash on unhandled promise rejections like Node.js does. If the answer is "no", then there's no issue here.

If the answer is "yes", then it depends on whether the callback we pass to onDeviceTokenRegistered gets executed on desktop or on web. My impression the ElectronBridge stuff is that it gets executed on web still, in which case there's still no issue here.

Depends on D10163

Test Plan: Flow; analysis in diff description

Reviewers: michal, marcin

Reviewed By: marcin

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10164