Custom shell completions overrides default cobra completions

We found out in https://src.fedoraproject.org/rpms/source-to-image/pull-request/9 that source-to-image has custom code in https://github.com/openshift/source-to-image/blob/master/pkg/cmd/cli/cmd/completion.go that overrides cobra's default completion subcommand. Maybe this was necessary with older versions of cobra, but now, overriding this removes the ability to generate completions for fish and powershell which cobra supports, and also the custom code does not use cobra's native support for zsh completions — it instead seems to borrow old code from kubectl. Can this custom completion command be removed?

Start with pkg/cmd/cli/cmd/completion.go and inspect how the custom completion subcommand replaces Cobra's native command. Remove the override if Cobra now provides the required behavior, then verify that completion generation works for fish, powershell, and zsh.