Drop separate testing for grpcio-gcp. (#5927)
Drop separate testing for grpcio-gcp. (#5927)

We made it an unconditional dependency.

See #5925.
dhermes/google-cloud-python,tseaver/gcloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,tseaver/gcloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,jonparrott/google-cloud-python,googleapis/google-cloud-python,jonparrott/google-cloud-python,jonparrott/gcloud-python,jonparrott/gcloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,dhermes/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python