Remove 'branches:' lines from .zuul.yaml
Remove 'branches:' lines from .zuul.yaml

These are no longer needed and complicate queue definition
because of inheritance across branches.  Also remove
unnecessary 'voting: false' lines for experimental jobs
since they are by definition non-voting.

Change-Id: Iad5ab15a8166298e5e317d6c822d42c6cbb6e8ab

openstack/manila,openstack/manila,bswartz/manila,bswartz/manila