bigquery: clean up parallel table.get for schema
bigquery: clean up parallel table.get for schema

Use a channel instead of a variable and a WaitGroup.

Change-Id: I3b8d1797ef87864f70c51559763ae964be8a4ae6
Reviewed-on: https://code-review.googlesource.com/15411
Reviewed-by: Sai Cheemalapati <36dc41adb0ea108898de35222b4f3585eb1d8968@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go