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