Allow subscriptions at org units without volumes

Allow subscriptions at org units without volumes

Subscriptions are sometimes owned at the consortium or system
level, so forcing can_have_vols() to be true was incorrect.

This commit removes those checks.  In addition, we also remove
some dead code and make the subscription org unit selector
consistent with the distribution org unit selector.

Signed-off-by: Dan Wells <dbw2@calvin.edu>