fix ClassCastException

Author: leyou240

dubbo-metrics/dubbo-tracing/src/main/java/org/apache/dubbo/tracing/handler/DubboClientTracingObservationHandler.java

@@ -20,7 +20,6 @@
 
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationHandler;
-import io.micrometer.observation.transport.SenderContext;
 import io.micrometer.tracing.Tracer;
 
 public class DubboClientTracingObservationHandler<T extends DubboClientContext> implements ObservationHandler<T> {

dubbo-metrics/dubbo-tracing/src/main/java/org/apache/dubbo/tracing/handler/DubboServerTracingObservationHandler.java

@@ -21,7 +21,6 @@
 
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationHandler;
-import io.micrometer.observation.transport.ReceiverContext;
 import io.micrometer.tracing.TraceContext;
 import io.micrometer.tracing.Tracer;