input: Write a python function to remove duplicate characters from a string.
  assert remove_duplicate_chars("Hello world") == {'H', 'e', 'l', 'o', ' ', 'w', 'r', 'd'}
  assert remove_duplicate_chars("Python programming") == {'P', 'y', 't', 'h', 'o', 'n', ' ', 'p', 'r', 'g', 'a', 'm', 'i'}
  assert remove_duplicate_chars("Mississippi") == {'M', 'i', 's', 'p'}
output: def remove_duplicate_chars(s):
      return set(s)