remove unused variables

weed/operation/grpc_client.go

@@ -2,20 +2,13 @@ package operation
 
 import (
 	"fmt"
-	"strconv"
-	"strings"
-	"sync"
-
 	"github.com/chrislusf/seaweedfs/weed/glog"
 	"github.com/chrislusf/seaweedfs/weed/pb/master_pb"
 	"github.com/chrislusf/seaweedfs/weed/pb/volume_server_pb"
 	"github.com/chrislusf/seaweedfs/weed/util"
 	"google.golang.org/grpc"
-)
-
-var (
-	grpcClients     = make(map[string]*grpc.ClientConn)
-	grpcClientsLock sync.Mutex
+	"strconv"
+	"strings"
 )
 
 func WithVolumeServerClient(volumeServer string, grpcDialOption grpc.DialOption, fn func(volume_server_pb.VolumeServerClient) error) error {