input: Write a python function to remove the last n characters from a given string.
  assert remove_last_n_characters("hello world", 3) == "hello wo"
  assert remove_last_n_characters("python", 2) == "pyth"
  assert remove_last_n_characters("data science", 6) == "data s"
output: def remove_last_n_characters(s, n):
      return s[:-n]