Do not ignore events deletion when events to be added are provided (#16916)

Kqueue/kevent implementation used to ignore events to be unsubscribed
from when events to be subscribed to were provided. This resulted in a
lost notification subscription, when GHC runtime didn't listen for any
events, yet the kernel considered otherwise and kept waking up the IO
manager thread.

This commit fixes this issue by always adding and removing all of the
provided subscriptions.