HUE-2449 [oozie] Remove redundant delete

This deletion is unnecessary as we're using the
django.middleware.transaction.TransactionMiddleware, wraps all
the view calls in a transaction, and roll it back if there is
an error. This also helps to avoid calling `coordinator.delete()`
if an exception happens before the coordinator was saved.
HUE-2449 [oozie] Raise an exception if oozie frequency is invalid