[starnix] Remove redundant receiver/sender

By updating `WrappedFuture` to be able to wrap any future and not only
those that have no return value, it is not necessary to use a
receiver/sender to extract result in the thread spawner.

Change-Id: I9b759d5953994daf15e3f0cfb8ff5aedc9ea6fec
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1450988
Reviewed-by: Kevin Lindkvist <lindkvist@google.com>
Fuchsia-Auto-Submit: Benjamin Lerman <qsr@google.com>
Commit-Queue: Benjamin Lerman <qsr@google.com>
2 files changed