Remove unused import

Signed-off-by: Mihai Maruseac <mihaimaruseac@google.com>

model_signing/serialization/serialize_by_file_shard.py

@@ -18,7 +18,7 @@
 import base64
 import concurrent.futures
 import pathlib
-from typing import Callable, Iterable, TypeAlias, cast
+from typing import Callable, Iterable, cast
 from typing_extensions import override
 
 from model_signing.hashing import file