input: Write a function to remove vowels from a given 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)