We no longer need this subcommand as the shell version calls the
'update' subcommand instead, which does all the cloning within C itself.
We also no longer need the 'update_clone_submodules()' and
'update_clone_submodule()' functions, so we remove those as well.
So in 04/13 update_clone_submodules() was renamed, but now we're getting
rid of it. Maybe there's not an easy way to avoid this churn, but if there is...