input: Write a python function to remove vowels from a 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])