No longer any need to pass admin context to aggregate DB API methods

No longer need to pass an elevated context because the check of
admin has been removed in following patch:
https://review.openstack.org/67026

This patch removes the admin context elevated
method in nova/scheduler sub-folder since it's not required any more.

Change-Id: Ib768c97c9e84a9db44d866fa5fbfc4665ad7c73e
Partial-Bug: #1279216
Merge "No longer any need to pass admin context to aggregate DB API methods"