ceph-dev-*-trigger: stop building master on bionic

ceph-dev-*-trigger: stop building master on bionic

in short, we don't need to test on bionic anymore for couple reasons:

- for accessing newer GCC
- for accessing more packages

see the discussion at
https://lists.ceph.io/hyperkitty/list/dev@ceph.io/thread/BW5NKQ4KTOPV7OYUBUHVDVCSH72LVIEO/

the only qa suite referencing ubuntu 18.04 is the qa/suites/orch/rook/smoke,
but it deploys ceph using Rook, and the container image of Ceph is
based on CentOS, so stop building on ubuntu/bionic does not
break it, despite the fact that 18.04 is included by
qa/suites/orch/rook/smoke/0-distro.

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