Drop asynctest (#44746)

tests/util/test_json.py

@@ -7,6 +7,7 @@ import os
 import sys
 from tempfile import mkdtemp
 import unittest
+from unittest.mock import Mock
 
 import pytest
 
@@ -19,8 +20,6 @@ from homeassistant.util.json import (
     save_json,
 )
 
-from tests.async_mock import Mock
-
 # Test data that can be saved as JSON
 TEST_JSON_A = {"a": 1, "B": "two"}
 TEST_JSON_B = {"a": "one", "B": 2}