chore: remove workarounds for BQ Storage issue with small result sets

[plamut]

Closes #106.

This PR removes workarounds the BQ Storage bug with small anonymous result sets that has been fixed recently, rendering those workarounds obsolete.

PR checklist

• 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)

[plamut]

Before this PR, a BigQuery Storage client was automatically created (a default) behind the scenes and actually hit the backend, but only to trigger a 401 Unauthorized error and fall back to the tabledata.list API that uses the (mocked) BigQuery client.

These create_bqstorage_client=False additions ensure that tests behave as before, i.e. using the (mocked) BigQuery client, but without a futile attempt with a real BQ Storage client.

[shollyman]

Thanks for clarifying

[plamut]

Some issue started occurring under Python 2.7 all of a sudden:

SyntaxError: Non-ASCII character '\xc3' in file /tmpfs/src/github/python-bigquery/.nox/unit-2-7/lib/python2.7/site-packages/rsa/key.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details

Seems like the rsa dependency is broken, a new version was released just half an hour ago - https://pypi.org/project/rsa/#history

Edit: Looking at the release notes, it seems that they dropped Python 2.7 support in version 4.1 already, but forgot to tag it as such. We need to pin rsa to an older version...

Edit 2: Will be fixed in #135.

[shollyman]

Thanks for cleaning this up.

[shollyman]

Thanks for clarifying