Using a Promise in BackgroundAction was not doing anything since the
change to use a weak reference to the event loop, so let's just drop
that.
The thread will now always move itself (and therefore its callbacks)
over to the originating thread before completing, regardless of the
presence of callbacks. This ensures that ref counting remains on the
main thread.
In addition, BackgroundAction's completion callback can no longer
return errors. This functionality wasn't actually used anywhere, it was
a holdover from the behavior of Core::Promise.
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