Cleanup requirements
Cleanup requirements

https://review.openstack.org/#/c/567830/ turns out that
trove-dashboard has horizon dependecies explicitly in requirements.txt
even if trove-dashboard does not use directly.
It makes the maintenance of requirements complicated,
so it would be better to drop them.

Change-Id: I8e1e6f4b5c4172df56a698a9091f1e98e9f6b6b8

openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard