db.backup_destroy requires admin context to mark backup record as
deleted. Commit Icff37261b367463b71a1268be16f9c97f595bf0c removed admin
context passed to backup_destroy method.
This patch uses elevated context to destroy backup.
Change-Id: I75b9e1fff48569a8aa320f2f02914fc7b6665d79
Closes-Bug: #1467167