tests/docker: Remove Debian 9 remnant lines

tests/docker: Remove Debian 9 remnant lines

Debian 9 base container has been removed in commits
e3755276d1f and c9d78b06c06. Remove the last remnants.

Fixes: e3755276d1f ("tests/docker: Remove old Debian 9 containers")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210107072933.3828450-1-f4bug@amsat.org>
Message-Id: <20210114165730.31607-2-alex.bennee@linaro.org>