input: Write a function to remove all vowels from a given string. The function should return the string with all vowels removed in both uppercase and lowercase cases.
  
  assert remove_vowels("Hello, World!") == "Hll, Wrld!"
  assert remove_vowels("Python Programming") == "Pythn Prgrmmng"
  assert remove_vowels("AEIOUaeiou") == ""
  assert remove_vowels("") == ""
  assert remove_vowels("QwRtY") == "QwRtY"
output: def remove_vowels(s):
      vowels = "AEIOUaeiou"
      return ''.join([char for char in s if char not in vowels])