Fix checkstyle errors

spring-test/src/test/java/org/springframework/test/web/reactive/server/HttpHandlerConnectorTests.java

@@ -28,7 +28,6 @@ import reactor.core.scheduler.Schedulers;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
-import org.springframework.core.testfixture.io.buffer.DataBufferTestUtils;
 import org.springframework.http.HttpCookie;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;

spring-webflux/src/test/java/org/springframework/web/reactive/function/BodyInsertersTests.java

@@ -45,7 +45,6 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.core.io.buffer.DefaultDataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
-import org.springframework.core.testfixture.io.buffer.DataBufferTestUtils;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpRange;
 import org.springframework.http.MediaType;

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -17,7 +17,6 @@
 package org.springframework.web.reactive.result.method.annotation;
 
 import java.net.URI;
-import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
@@ -41,7 +40,6 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.ByteBufferEncoder;
 import org.springframework.core.codec.CharSequenceEncoder;
-import org.springframework.core.testfixture.io.buffer.DataBufferTestUtils;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;