input: Write a function to filter out all the vowels from a given string. The function should return the string with all vowels removed. If the input string is empty, the function should return an empty string.
  assert filter_vowels("hello")=="hll"
  assert filter_vowels("world")=="wrld"
  assert filter_vowels("aeiou")==""
  assert filter_vowels("python")=="pythn"
  assert filter_vowels("")==""
output: def filter_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])