chore: remove migrated snippets

[dizcology]

The snippets have been migrated to python-docs-samples (GoogleCloudPlatform/python-docs-samples#10261) and documentation pages updated (example: https://cloud.google.com/bigquery/docs/reservations-tasks#create_a_reservation_with_dedicated_slots).

[snippet-bot]

You are about to delete the following sample browser page.

• samples/snippets/quickstart.py:15, tag bigqueryreservation_quickstart(usage: page 1).

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Here is the summary of changes.

You are about to delete 4 region tags.

• samples/snippets/quickstart.py:15, tag bigqueryreservation_quickstart
• samples/snippets/reservation_create.py:31, tag bigqueryreservation_reservation_create
• samples/snippets/reservation_delete.py:24, tag bigqueryreservation_reservation_delete
• samples/snippets/reservation_update.py:31, tag bigqueryreservation_reservation_update

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