[mypy] fix type annotations for other/least-recently-used.py (#5811)

Author: pikulet

Diff for: other/least_recently_used.py

@@ -1,5 +1,4 @@
 import sys
-from abc import abstractmethod
 from collections import deque
 
 
@@ -10,7 +9,6 @@ class LRUCache:
     key_reference_map = object()  # References of the keys in cache
     _MAX_CAPACITY: int = 10  # Maximum capacity of cache
 
-    @abstractmethod
     def __init__(self, n: int):
         """Creates an empty store and map for the keys.
         The LRUCache is set the size n.