all: remove os.ErrTemporary

As discussed in
https://github.com/golang/go/issues/32463#issuecomment-506833421
the classification of deadline-based timeouts as "temporary" errors is a
historical accident. I/O timeouts used to be duration-based, so they
really were temporary--retrying a timed-out operation could succeed. Now
that they're deadline-based, timeouts aren't temporary unless you reset
the deadline.

Drop ErrTemporary from Go 1.13, since its definition is wrong. We'll
consider putting it back in Go 1.14 with a clear definition and
deprecate net.OpError.Temporary.

Fixes #32463

Change-Id: I70cda664590d8872541e17409a5780da76920891
Reviewed-on: https://go-review.googlesource.com/c/go/+/188398


Reviewed-by: Jonathan Amsterdam <jba@google.com>