chore(spanner): remove unused asyncio import in samples (googleapis#17620)

Remove unused `import asyncio` from `async_snippets.py` to fix lint
failure.

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Fixes #<issue_number_goes_here> 🦕

Author: sakthivelmanii

packages/google-cloud-spanner/samples/samples/async_snippets.py

@@ -18,8 +18,6 @@
 Cloud Spanner.
 """
 
-import asyncio
-
 from google.cloud.spanner_v1 import AsyncClient, KeySet