Fix the span cannot stop when forward request (#2820)

apm-sniffer/apm-sdk-plugin/tomcat-7.x-8.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/tomcat78x/ForwardInterceptor.java

@@ -46,7 +46,7 @@ public class ForwardInterceptor implements InstanceMethodsAroundInterceptor, Ins
     @Override
     public Object afterMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
         Object ret) throws Throwable {
-
+        ContextManager.getRuntimeContext().remove(Constants.FORWARD_REQUEST_FLAG);
         return ret;
     }