fix: remove default timeout (#974)

Internal folks, see: go/microgenerator-retries

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

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Fixes #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