input: Write a function to remove all vowels from a given string. If the input string is empty, then return an empty string.
  assert remove_vowels("hello world")=="hll wrld"
  assert remove_vowels("aeiouAEIOU")==""
  assert remove_vowels("python programming")=="pythn prgrmmng"
  assert remove_vowels("This is a test")=="Ths s  tst"
  assert remove_vowels("")==""
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])