Drop legacy-horizon-dsvm-tempest-plugin
Drop legacy-horizon-dsvm-tempest-plugin

The legacy job has been converted into
horizon-dsvm-tempest-plugin in openstack/horizon repo
(https://review.openstack.org/#/c/519084/).

We can now drop the legacy horizon-dsvm-tempest-plugin
from horizon and tempest-horizon gates.

Needed-By: Id9371caea84b964505e7ba1e18ecbedc1d9ad825
Change-Id: I8ac2f9098b698c14de131c25b77e1110a84a770d

openstack-infra/project-config,openstack-infra/project-config