Tracker for cleanups possible after RHCOS moves to FCOS pipeline

This is a tracker for all the cleanups we should be able to do once we've fully moved over RHCOS to the new pipeline.

• delete cosa push-container; we only use cosa push-container-manifest now for all our containers
• delete upload-ami; we use pure cosa buildextend-aws --upload now with support for granting to additional users, scheduling AWS tests and marking the AMIs public: https://github.com/openshift/os/pull/1074
• delete cosa buildextend-extensions; this was the legacy way to publish extensions
• make digest required again in images (see https://github.com/coreos/coreos-assembler/pull/3129) (though this is more dependent on branching on 4.12)
• delete cosa build-extensions-container backcompat alias
• delete cosa buildextend-legacy-oscontainer; we don't need to build or ship the legacy oscontainer anymore

Start by locating the implementations or command definitions for cosa push-container, cosa buildextend-extensions, cosa build-extensions-container, and cosa buildextend-legacy-oscontainer. Check the remaining unchecked items against the completed pipeline migration and the referenced digest dependency; done means the obsolete commands and aliases are removed and the checklist's applicable cleanup is complete.