input: Write a python function to create a new string by omitting characters in the second string that are present in the first string.
  assert new_string("Python", "Programming") == "roammig"
  assert new_string("hello", "world") == "wrd"
  assert new_string("abc", "def") == "def"
output: def new_string(str1, str2):
      return ''.join([char for char in str2 if char not in str1])