input: Write a python function to remove a specified number from a list.
  assert remove_num([1, 7, 3, 1, 4, 7, 9], 7) == [1, 3, 1, 4, 9]
  assert remove_num([10, 20, 30, 40, 50], 30) == [10, 20, 40, 50]
  assert remove_num([5, 5, 5, 5, 5], 5) == []
output: def remove_num(lst, num):
      return [x for x in lst if x != num]