[release-branch.go1.25] 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.

For #76961
Fixes #76973

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>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 1b3db48db7)
Reviewed-on: https://go-review.googlesource.com/c/go/+/734520
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/errors/join.go

@@ -26,18 +26,6 @@ func Join(errs ...error) error {
 	if n == 0 {
 		return nil
 	}
-	if n == 1 {
-		for _, err := range errs {
-			if err != nil {
-				if _, ok := err.(interface {
-					Unwrap() []error
-				}); ok {
-					return err
-				}
-			}
-		}
-	}
-
 	e := &joinError{
 		errs: make([]error, 0, n),
 	}

src/errors/join_test.go

@@ -70,37 +70,3 @@ func TestJoinErrorMethod(t *testing.T) {
 		}
 	}
 }
-
-func BenchmarkJoin(b *testing.B) {
-	for _, bb := range []struct {
-		name string
-		errs []error
-	}{
-		{
-			name: "no error",
-		},
-		{
-			name: "single non-nil error",
-			errs: []error{errors.New("err")},
-		},
-		{
-			name: "multiple errors",
-			errs: []error{errors.New("err"), errors.New("newerr"), errors.New("newerr2")},
-		},
-		{
-			name: "unwrappable single error",
-			errs: []error{errors.Join(errors.New("err"))},
-		},
-		{
-			name: "nil first error",
-			errs: []error{nil, errors.New("newerr")},
-		},
-	} {
-		b.Run(bb.name, func(b *testing.B) {
-			b.ReportAllocs()
-			for i := 0; i < b.N; i++ {
-				_ = errors.Join(bb.errs...)
-			}
-		})
-	}
-}