Code:

---

text temp_txt
{
  strings = 1;
}

function del_last_char(_txt)
{
  var handle;

  //load the txt into a text object
  txt_load(temp_txt,_txt);

  //wait till its loaded
  wait(3);

  //cut off the last char
  str_trunc(temp_txt.string[0],1);
  
  //now resave the file:
  handle = file_open_write(_txt);
  file_str_write(handle,temp_txt.string[0]);
  file_close(handle);
}

//call the function like this:
del_last_char("my_text_file.txt");

---

This is NOT tested but should work.