fix: remove default timeout (googleapis#974)

Internal folks, see: go/microgenerator-retries

> "Methods will **not** hedge by default." (emphasis mine)

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](https://github.com/googleapis/python-bigquery/issues/new/choose) 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 googleapis#970 🦕

google/cloud/bigquery/retry.py

@@ -60,7 +60,7 @@ def _should_retry(exc):
 pass ``retry=bigquery.DEFAULT_RETRY.with_deadline(30)``.
 """
 
-DEFAULT_TIMEOUT = 5.0 * 60.0
+DEFAULT_TIMEOUT = None
 """The default API timeout.
 
 This is the time to wait per request. To adjust the total wait time, set a