Update sleap/nn/tracking.py

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

sleap/nn/tracking.py

@@ -3,7 +3,7 @@
 import abc
 import json
 import sys
-from collections import deque, defaultdict
+from collections import deque
 from time import time
 from typing import Callable, Deque, Dict, Iterable, List, Optional, Tuple