fix flake8

Author: rahul2393

tests/system/test_backup_api.py

@@ -19,7 +19,6 @@
 import pytest
 
 from google.api_core import exceptions
-from google.cloud import spanner_v1
 from . import _helpers
 
 skip_env_reason = f"""\

tests/system/test_dbapi.py

@@ -19,7 +19,6 @@
 import time
 import decimal
 
-from google.cloud import spanner_v1
 from google.cloud._helpers import UTC
 
 from google.cloud.spanner_dbapi.connection import Connection, connect