Remove redundant @ExtendWith(SpringExtension.class) for sample

See gh-32476

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/testing/springbootapplications/jmx/MyJmxTests.java

@@ -19,16 +19,13 @@
 import javax.management.MBeanServer;
 
 import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.extension.ExtendWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.test.annotation.DirtiesContext;
-import org.springframework.test.context.junit.jupiter.SpringExtension;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-@ExtendWith(SpringExtension.class)
 @SpringBootTest(properties = "spring.jmx.enabled=true")
 @DirtiesContext
 class MyJmxTests {