qa/tests - removed ref to 18.04 distro as it's not supported on master+

Signed-off-by: Yuri Weinstein <yweinste@redhat.com>

diff --git a/qa/suites/orch/cephadm/smoke/distro/ubuntu_18.04.yaml b/qa/suites/orch/cephadm/smoke/distro/ubuntu_18.04.yaml
deleted file mode 120000 (symlink)
index cfb85f1..0000000
--- a/qa/suites/orch/cephadm/smoke/distro/ubuntu_18.04.yaml
+++ /dev/null
@@ -1 +0,0 @@
-.qa/distros/all/ubuntu_18.04.yaml
\ No newline at end of file