The Debian 9 containers have been removed a while ago, so we can
delete the corresponding entries in the Makefile, too.

Fixes: e3755276 ("tests/docker: Remove old Debian 9 containers")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20201215083318.92205-1-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>