bots: Remove nightly.repo for rhel-7-6 image
bots: Remove nightly.repo for rhel-7-6 image

It does not exist any more.  This caused lorax-composer to fail to start
up.

Closes #10902

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