Update 212-Word-Search-II.py

[AgranatMarkit]

Added following optimizations to bypass "Time Limit Exceeded":

1. when we add word, we can unmark node in trie to prevent redundant hashing checks in our words set. node.isWord = false
2. in the previous case we also should remove a word from the Trie. To prevent redundant heap deallocations, it's better to "mark" an Trie node using refs property. If refs == 0, then node is deleted.