delete not use import

Author: chengyouling

handlers/handler-loadbalance/src/main/java/org/apache/servicecomb/loadbalance/filterext/IsolationServerListFilterExt.java

@@ -21,7 +21,6 @@
 import java.util.List;
 
 import org.apache.servicecomb.core.Invocation;
-import org.apache.servicecomb.foundation.common.Holder;
 import org.apache.servicecomb.foundation.common.event.AlarmEvent.Type;
 import org.apache.servicecomb.foundation.common.event.EventManager;
 import org.apache.servicecomb.loadbalance.Configuration;