input: Write a python function to remove all vowels from a given string.
  assert remove_vowels('Hello World!') == 'Hll Wrld!'
  assert remove_vowels('aeiou') == ''
  assert remove_vowels('Python Programming') == 'Pythn Prgrmmng'
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])