input: Write a function to remove all vowels from a given string. Return the new string with vowels removed.
  assert remove_vowels("remove the vowels")=="rmv th vwls"
  assert remove_vowels("HELLO WORLD")=="HLLO WRLD"
  assert remove_vowels("Python Programming")=="Pythn Prgrmmng"
  assert remove_vowels("")==""
  assert remove_vowels("aEiOu")==""
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])