all: remove os.ErrTimeout

It is unclear whether the current definition of os.IsTimeout is
desirable or not. Drop ErrTimeout for now so we can consider adding it
(or some other error) in a future release with a corrected definition.

Fixes #33411

Change-Id: I8b880da7d22afc343a08339eb5f0efd1075ecafe
Reviewed-on: https://go-review.googlesource.com/c/go/+/188758


Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>