Fix checkstyle violations

Author: rstoyanchev

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultServerResponseBuilder.java

@@ -37,7 +37,6 @@
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.core.codec.Hints;
 import org.springframework.http.CacheControl;
-import org.springframework.http.ETag;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatusCode;

spring-webmvc/src/main/java/org/springframework/web/servlet/function/DefaultServerResponseBuilder.java

@@ -31,7 +31,6 @@
 
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.http.CacheControl;
-import org.springframework.http.ETag;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatusCode;