Fix checkstyle issue

Author: rstoyanchev

spring-graphql/src/test/java/org/springframework/graphql/data/method/annotation/support/DataFetcherHandlerMethodTests.java

@@ -28,7 +28,6 @@
 import graphql.schema.DataFetchingEnvironmentImpl;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
-import reactor.test.StepVerifier;
 
 import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.graphql.data.GraphQlArgumentBinder;