spanner: complete benchserver removal

Removes non-existent and no longer removed
imports since benchserver was removed in
CL https://code-review.googlesource.com/c/gocloud/+/45351.

Change-Id: Idfd83d144d655bb1716dfbeedbdbebff028e6d9f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/45371
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/spanner/client_test.go b/spanner/client_test.go
index e8e5fc8..fa36de6 100644
--- a/spanner/client_test.go
+++ b/spanner/client_test.go
@@ -20,12 +20,10 @@
 	"context"
 	"fmt"
 	"io"
-	"os"
 	"strings"
 	"testing"
 
 	itestutil "cloud.google.com/go/internal/testutil"
-	"cloud.google.com/go/spanner/internal/benchserver"
 	. "cloud.google.com/go/spanner/internal/testutil"
 	"google.golang.org/api/iterator"
 	"google.golang.org/api/option"