Refactor SockJsFrame by removing unused imports

Signed-off-by: KNU-K <[email protected]>

Author: polyglot-k

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/SockJsFrame.java

@@ -18,11 +18,8 @@
 
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
-
 import org.jspecify.annotations.Nullable;
-
 import org.springframework.util.Assert;
-import org.springframework.util.StringUtils;
 
 /**
  * Represents a SockJS frame. Provides factory methods to create SockJS frames.