chore: move bigframes package to separate repo

[tswast]

https://github.com/google/bigframes is the new location for bigframes.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• packages/bigframes/samples/polars/create_polars_df_with_to_arrow_test.py:17, tag bigquery_dataframes_to_polars

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[gemini-code-assist]

Code Review

This pull request completely removes the bigframes package from this repository, including all source code, tests, configuration files, Kokoro CI/CD configurations, and release manifests. References to bigframes in the root README.rst have been updated to point to its new standalone repository on GitHub. As there are no review comments provided, I have no feedback to offer.