fix: remove unused ClientContext imports to pass flake8 lint

Author: aseering

google/cloud/spanner_v1/client.py

@@ -50,7 +50,6 @@
 from google.cloud.spanner_v1 import __version__
 from google.cloud.spanner_v1 import ExecuteSqlRequest
 from google.cloud.spanner_v1 import DefaultTransactionOptions
-from google.cloud.spanner_v1.types import ClientContext
 from google.cloud.spanner_v1._helpers import (
     _create_experimental_host_transport,
     _merge_query_options,

google/cloud/spanner_v1/snapshot.py

@@ -55,7 +55,6 @@
 from google.cloud.spanner_v1._opentelemetry_tracing import trace_call, add_span_event
 from google.cloud.spanner_v1.streamed import StreamedResultSet
 from google.cloud.spanner_v1 import RequestOptions
-from google.cloud.spanner_v1.types import ClientContext
 
 from google.cloud.spanner_v1.metrics.metrics_capture import MetricsCapture
 from google.cloud.spanner_v1.types import MultiplexedSessionPrecommitToken