input: Write a python function to remove vowels from a given phrase.
  assert remove_vowels("hello world") == "hll wrld"
  assert remove_vowels("programming is fun") == "prgrmmng s fn"
  assert remove_vowels("aeiou") == ""
output: def remove_vowels(phrase):
      vowels = "aeiouAEIOU"
      return ''.join(char for char in phrase if char not in vowels)