admin: remove ::sudo in favor of sudo::user/group

Now that we've fully converged the two modules, there's really no point
in duplicating this sort of functionality in two places; it's confusing
already (as evident by previous commits) and wouldn't be DRY.

Make the sudo::user/group definitions canonical and remove admin::sudo
altogether.

Change-Id: I4745c83b1357a764c066f5aa74ef3e2f9799e84a