Remove comments and to-do for quota inconsistency

There are comments for quota inconsistency in function
_complete_deletion and considering race conditions.

_complete_deletion was called by _complete_partial_deletion
and_delete_instance only if the instance state is DELETED
compute can enter _complete_partial_deletion
while if _complete_deletion called in _delete_instance,
that means instance vm state already become DELETED

also, if the _delete_instance was called with instance
state SOFT_DELETED, the quotas.rollback() will be called
it will make quotas.commit later take no effect
since the quotas.reservations is already None

so there is no race conditions and the comments can be removed

Change-Id: Iddb9e5197ca7ca9bd4692d63bb5c7f9a2ab44be5
Partial-Bug: #1296414
Merge "Remove comments and to-do for quota inconsistency"