input: Write a python function to clean a string by removing specified special characters.
  assert clean_string("He*llo& Wo#rld!@") == "Hello World"
  assert clean_string("*Pyt$hon@#") == "Python"
  assert clean_string("No*Speci&als") == "NoSpecials"
output: def clean_string(input_string):
      special_characters = "*&@#$"
      return ''.join(char for char in input_string if char not in special_characters)