input: Write a function to remove all occurrences of a substring from a given string.
  assert remove_substring("I like cats, cats and cats", "cats") == "I like ,  and "
  assert remove_substring("hello world, hello universe", "hello") == " world,  universe"
  assert remove_substring("repeat repeat repeat", "repeat") == "  "
output: def remove_substring(s: str, sub: str) -> str:
      return s.replace(sub, "")