input: Write a function that removes all vowels from a given string and returns the modified string. 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])