input: Write a function to remove vowels from a given string.
  assert remove_vowels("hello world") == "hll wrld"
  assert remove_vowels("Python is fun") == "Pythn s fn"
  assert remove_vowels("AEIOUaeiou") == ""
output: def remove_vowels(s):
      vowels = 'aeiouAEIOU'
      return ''.join(char for char in s if char not in vowels)