input: Write a python function that receives a string as input and outputs the string with all vowels removed.
  assert remove_vowels('') == ''
  assert remove_vowels("abcdef\nghijklm") == 'bcdf\nghjklm'
  assert remove_vowels('fedcba') == 'fdcb'
  assert remove_vowels('eeeee') == ''
  assert remove_vowels('acBAA') == 'cB'
  assert remove_vowels('EcBOO') == 'cB'
  assert remove_vowels('ybcd') == 'ybcd'
output: def remove_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join(char for char in s if char not in vowels)