No you cannot use the UploadFileFTP to delete a file from an FTP server.
The function has 2 purposes:
1) To check to make sure the credentials are correct to login (this is done by not passing a file path)
2) To upload a file.
You would need to use the Win32 API for FTP functions but the callback is an asynchronus callback and not recommended to be used in Messenger Plus!.
Recommendation: Create an ActiveXObject or C++ DLL to do what you need to do.