chore: remove unused imports from test file

Remove unused json and Thread imports flagged by ruff linter.

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Co-Authored-By: Claude <[email protected]>

Author: gustavohstrassburger

posthog/test/test_realtime_feature_flags.py

@@ -1,7 +1,5 @@
-import json
 import time
 import unittest
-from threading import Thread
 from unittest import mock
 
 from posthog.client import Client