pubsub: run `go mod tidy`
pubsub: run `go mod tidy`

Change-Id: I925c3ea212ca60d25519c5727c3c1c664be40bbe
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/44890
Reviewed-by: Alex Hong <3311379d571f0e1dc66d44c9732cbfa024b0fda6@google.com>

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