src: ensure no more platform foreground tasks after Deinit

Node first calls `Isolate::Dispose`, then
`NodePlatform::UnregisterIsolate`.
This again calls `PerIsolatePlatformData::Shutdown`, which (before this
patch) called `FlushForegroundTasksInternal`, which might call
`RunForegroundTask` if it finds foreground tasks to be executed. This
will fail however, since `Isolate::GetCurrent` was already reset during
`Isolate::Dispose`.
Hence remove the check to `FlushForegroundTasksInternal` and add checks
instead that no more foreground tasks are scheduled.

Refs: https://github.com/v8/node/pull/86

PR-URL: https://github.com/nodejs/node/pull/25653


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>