🐛 fix(exam): 移除提交记录中的冗余nickname字段

- 从SubmissionRecords中删除nickname属性，避免数据冗余
- 提交记录不再存储用户名信息，优化数据结构

src/server/socket/services/exam.service.ts

@@ -204,7 +204,6 @@ export class ExamService {
         const submissionRecord = new SubmissionRecords();
         submissionRecord.classroomNo = roomId;
         submissionRecord.userId = answer.userId?.toString() || null;
-        submissionRecord.nickname = answer.username || null;
         submissionRecord.score = this.calculateScore(answer);
         submissionRecord.code = answer.code || null;
         submissionRecord.trainingDate = answer.date ? new Date(answer.date) : null;