Fixes for third-party mock package (#6685)

Removed a few unused private classes and methods. They can be re-added if a user reports them missing.

pyrightconfig.stricter.json

@@ -38,6 +38,7 @@
         "stubs/jsonschema",
         "stubs/ldap3",
         "stubs/Markdown",
+        "stubs/mock",
         "stubs/mysqlclient",
         "stubs/oauthlib",
         "stubs/Pillow",