Fix checkstyle violations

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/upgrade/JettyRequestUpgradeStrategy.java

@@ -18,7 +18,6 @@ package org.springframework.web.reactive.socket.server.upgrade;
 
 import java.io.IOException;
 import java.util.function.Supplier;
-
 import javax.servlet.ServletContext;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/upgrade/TomcatRequestUpgradeStrategy.java

@@ -19,7 +19,6 @@ package org.springframework.web.reactive.socket.server.upgrade;
 import java.io.IOException;
 import java.util.Collections;
 import java.util.function.Supplier;
-
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/upgrade/UndertowRequestUpgradeStrategy.java

@@ -35,7 +35,6 @@ import org.springframework.http.server.reactive.AbstractServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpRequestDecorator;
 import org.springframework.lang.Nullable;
-import org.springframework.util.Assert;
 import org.springframework.web.reactive.socket.HandshakeInfo;
 import org.springframework.web.reactive.socket.WebSocketHandler;
 import org.springframework.web.reactive.socket.adapter.UndertowWebSocketHandlerAdapter;