std.Io.Threaded: remove handling of ECANCELED

The only known situation in which this occurs is when using musl with
the undocumented extension PTHREAD_CANCEL_MASKED, causing the next
syscall to return ECANCELED.

However zig std lib does not use this mechanism, even when targeting
musl libc because it would require each async task to be on a fresh
pthread.

For the same reason, if third party code were to cause ECANCELED to be
returned from any of these syscalls, it would cause subsequent tasks to
be incorrectly canceled since they cannot be rearmed.

Thus, this Io implementation cannot handle this error code correctly,
expecting never to receive it.

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