qa/suites/orch/rook/smoke: stop testing on ubuntu 18.04

qa/suites/orch/rook/smoke: stop testing on ubuntu 18.04

even rook does not really install ceph packages in the host directly, it
uses the ceph container image. but teuthology insists on checking the
existence of debian packages by querying shaman server when it sees a
teuthology facet file which includes:

os_type: ubuntu
os_version: "18.04"

but since we've stopped building ubuntu/bionic packages, teuthology
just complains when we are scheduling test suites which are composed
from facets in qa/suites/orch/rook/smoke.

in this change, the ubuntu_18.04.yaml is dropped because ubuntu/bionic
does not really increase the test coverage of ceph. it helps to test
the rook and container runtime though.

Signed-off-by: Kefu Chai <kchai@redhat.com>