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("This is a test.")== "Ths s  tst."
  assert remove_vowels("Hello, World!")== "Hll, Wrld!"
  assert remove_vowels("")== ""
  assert remove_vowels("AeIoU")== ""
  assert remove_vowels("Python Programming")== "Pythn Prgrmmng"
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])