pubsub: fix duplicate import
pubsub: fix duplicate import

Change-Id: I69ea40d757dd80b8b7768fda781a64c7ccbf2c52
Reviewed-on: https://code-review.googlesource.com/14230
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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