tests/docker: Remove old Debian 9 containers

We do not support Debian 9 in QEMU anymore, and the Debian 9 containers
are now no longer used in the gitlab-CI. Time to remove them.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200921174320.46062-6-thuth@redhat.com>
Message-Id: <20200925154027.12672-14-alex.bennee@linaro.org>