Fix Checkstyle

servlet/spring-boot/java/data/src/main/java/example/User.java

@@ -16,7 +16,6 @@
 
 package example;
 
-import com.fasterxml.jackson.databind.JsonSerializer;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import jakarta.persistence.Entity;
 import jakarta.persistence.Id;