message: Update lru-cache.py
  I can't explain why, but with 'del node', the following code will go into an infinite loop:
  l = LinkedList()
  one=ListNode('a',1)
  two=ListNode('b',2)
  three=ListNode('c',3)
  four=ListNode('d',4)
  l.insert(one)
  l.insert(two)
  l.insert(three)
  l.insert(four)
  print (l)
  l.remove(three)
  print (l)
  l.insert(three)
  print (l)
  l.remove(one)
  print (l)
  l.remove(three)
  print (l)
diff: @@ -55,7 +55,8 @@ class LinkedList(object):
  node.next.prev = node.prev
  else:
  self.tail = node.prev
  - del node
  + node.next = None
  + node.prev = None
  class LRUCache(object):
  