[XCI] Remove inventory for prepare-tests.yml

We are finally fetching all ODL variables from inside the scenarios.
Therefore, we don't need to use the inventory:

${XCI_PATH}/.cache/repos/openstack-ansible/inventory/dynamic_inventory.py

Change-Id: I91d17cb899a037887bc97441a1c1e37ff57b4850
Signed-off-by: Manuel Buil <mbuil@suse.com>