Remove unused elevated context param from quota helper methods

The elevated context being passed to the _quota_usage_create and
_reservation_create methods is not used, so remove it from the formal
parameter list of those methods and related usage.

This is part of a larger series to refactor the quota_reserve method.

Partial-Bug: #1369605

Change-Id: I50749402177eda29ae006ec4fd11a9a8125765ec
Merge "Remove unused elevated context param from quota helper methods"