input: Write a function to remove elements from a list that exist in another list.
  assert remove_elements([1, 2, 3, 4, 5], [4, 5, 6])==[1, 2, 3]
  assert remove_elements([10, 20, 30], [20, 40])==[10, 30]
  assert remove_elements([7, 8, 9], [8, 9, 10])==[7]
output: def remove_elements(lst1, lst2):
      return [item for item in lst1 if item not in lst2]