Revert "errors: optimize errors.Join for single unwrappable errors"
This reverts CL 635115.
Reason for revert: The new behavior does not match the function documentation.
Fixes #76961
Change-Id: If2450aa4efba28c7a12887a5b306c231a836e740
Reviewed-on: https://go-review.googlesource.com/c/go/+/731981 Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
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