Previously we were using .sk.CloseRead() to interrupt sk.Read(), but
that is not necessary since .sk, relying on xio.Pipe, implements
xio.Reader natively with full support for cancellation.

The original code to cancel via CloseRead comes from mid 2019 and predates

kirr/go123@7ad867a3
kirr/go123@0e368363
kirr/go123@0bdac628
kirr/go123@9db4dfac
kirr/go123@d2dc6c09

And in kirr/wendelin.core@b17aeb8c and
6f0cdaff (wcfs: Provide isolation to clients), it seems, I missed to
update WatchLink.serve code to that.

Do that now because it simplifies code flow organization a bit.

/reviewed-by @levin.zimmermann
/reviewed-on !18