del function

## 목적
삭제 기능 추가
## 작업 상세 내용
main.cpp, data.h, data.cpp에 삭제 기능인 del을 추가하였습니다.
## Reference

The issue mentions adding a delete function 'del' to main.cpp, data.h, and data.cpp. Start by examining the existing structure in data.h and data.cpp to understand how data is stored and managed. Look for related functions like add or update to follow the existing patterns. Implement the delete logic, ensuring it handles edge cases, and test it by running the compiled program to verify the deletion works correctly.

索引モデルが issue の本文から書いたものです。