chore: remove unused code snippets

[tswast]

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Fixes internal issue 199162556 🦕

Blocked by CL 428614857

[snippet-bot]

Here is the summary of changes.

You are about to delete 2 region tags.

• docs/snippets.py:770, tag bigquery_cancel_job
• docs/snippets.py:780, tag bigquery_get_job

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