input: Write a function to remove all vowels from a given string. If the given string is empty then return an empty string.
  assert remove_vowels("Hello, World!")=="Hll, Wrld!"
  assert remove_vowels("Python Programming")=="Pythn Prgrmmng"
  assert remove_vowels("AEIOUaeiou")==""
  assert remove_vowels("12345")=="12345"
  assert remove_vowels("")==""
output: def remove_vowels(s):
      vowels = "AEIOUaeiou"
      return ''.join([char for char in s if char not in vowels])