Remove implements TestRule from EmbeddedKafkaRule

`ExternalResource` already implements `TestRule` which makes 
an explicit implementation of `TestRule` obsolete.

Author: Treppenhouse

spring-kafka-test/src/main/java/org/springframework/kafka/test/rule/EmbeddedKafkaRule.java

@@ -32,7 +32,7 @@
  *
  * @see EmbeddedKafkaBroker
  */
-public class EmbeddedKafkaRule extends ExternalResource implements TestRule {
+public class EmbeddedKafkaRule extends ExternalResource {
 
 	private final EmbeddedKafkaBroker embeddedKafka;