input: Write a function to remove vowels from a string.
  assert remove_vowel('hello') == 'hll'
  assert remove_vowel('beautiful') == 'bttf'
  assert remove_vowel('python') == 'pythn'
output: def remove_vowel(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])