Remove build-openstack-sphinx-docs-python3
Remove build-openstack-sphinx-docs-python3

This obsolete template is not used anymore, remove it.

Change-Id: Ic96550073c2d4f3645eec519ded26427249c4eb1
Depends-On: https://review.openstack.org/609792

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs