input: Write a function to remove all vowels from a given string. The function should return the modified string with all vowels removed. If the input string is empty, the function should return an empty string.
  assert remove_vowels("hello world")=="hll wrld"
  assert remove_vowels("Python Programming")=="Pythn Prgrmmng"
  assert remove_vowels("Aeiou")==""
  assert remove_vowels("")==""
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])