docs: remove migrated snippets

[dizcology]

The snippets have been migrated (GoogleCloudPlatform/python-docs-samples#10259) and the documentation pages have been updated (example: https://cloud.google.com/bigquery/docs/working-with-transfers#list_transfers)

[snippet-bot]

Here is the summary of changes.

You are about to delete 20 region tags.

• samples/snippets/copy_dataset.py:17, tag bigquerydatatransfer_copy_dataset
• samples/snippets/copy_dataset.py:37, tag bigquerydatatransfer_copy_dataset
• samples/snippets/manage_transfer_configs.py:17, tag bigquerydatatransfer_list_configs
• samples/snippets/manage_transfer_configs.py:27, tag bigquerydatatransfer_list_configs
• samples/snippets/manage_transfer_configs.py:38, tag bigquerydatatransfer_update_config
• samples/snippets/manage_transfer_configs.py:53, tag bigquerydatatransfer_update_config
• samples/snippets/manage_transfer_configs.py:73, tag bigquerydatatransfer_update_credentials
• samples/snippets/manage_transfer_configs.py:90, tag bigquerydatatransfer_update_credentials
• samples/snippets/manage_transfer_configs.py:109, tag bigquerydatatransfer_schedule_backfill
• samples/snippets/manage_transfer_configs.py:128, tag bigquerydatatransfer_schedule_backfill
• samples/snippets/manage_transfer_configs.py:165, tag bigquerydatatransfer_delete_transfer
• samples/snippets/manage_transfer_configs.py:178, tag bigquerydatatransfer_delete_transfer
• samples/snippets/quickstart.py:21, tag bigquerydatatransfer_quickstart
• samples/snippets/quickstart.py:32, tag bigquerydatatransfer_quickstart
• samples/snippets/run_notification.py:19, tag bigquerydatatransfer_run_notification
• samples/snippets/run_notification.py:26, tag bigquerydatatransfer_run_notification
• samples/snippets/scheduled_query.py:17, tag bigquerydatatransfer_create_scheduled_query
• samples/snippets/scheduled_query.py:18, tag bigquerydatatransfer_create_scheduled_query_with_service_account
• samples/snippets/scheduled_query.py:41, tag bigquerydatatransfer_create_scheduled_query
• samples/snippets/scheduled_query.py:42, tag bigquerydatatransfer_create_scheduled_query_with_service_account

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