openstack-full.install: Remove duplicate backports

openstack-full > openstack-common > cloud

We already have backports declared in cloud.install

https://github.com/enovance/edeploy-roles/blob/master/cloud.install#L39-L41

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
Merge pull request #109 from dsavineau/fix-duplicate-backports

openstack-full.install: Remove duplicate backports