bots: Drop obsolete jessie-backports repo on debian-testing image
bots: Drop obsolete jessie-backports repo on debian-testing image

docker.io is in testing again with a much newer version, so nothing
comes from the Debian 8 backports repo any more.

martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,andreasn/cockpit,deryni/cockpit,mareklibra/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,mareklibra/cockpit,cockpituous/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpit-project/cockpit,mareklibra/cockpit,mareklibra/cockpit,cockpituous/cockpit,garrett/cockpit,andreasn/cockpit,andreasn/cockpit,martinpitt/cockpit,andreasn/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,andreasn/cockpit,martinpitt/cockpit,mareklibra/cockpit,mvollmer/cockpit