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("")==""
  assert remove_vowels("aeiouAEIOU")==""
  assert remove_vowels("12345")=="12345"
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])