Removing redundant constant. (#4588)
Removing redundant constant. (#4588)

It looks like it was moved to `google.api_core.gapic_v1.client_info`
and during the move the delete was forgotten.
GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,tseaver/gcloud-python,dhermes/gcloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python,googleapis/google-cloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,tseaver/gcloud-python,jonparrott/gcloud-python,dhermes/gcloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,tswast/google-cloud-python,tswast/google-cloud-python,jonparrott/google-cloud-python,tseaver/google-cloud-python,jonparrott/google-cloud-python