Remove notifier_strategy config from glance-api

Glance now uses notification_driver. See Glance commit
90d6ef8130fd7e121e35221eb1452e6646ba4047 or change ID:

 I8cd84772bc5867e06b2a50ed7e15b9e86f0b94ad

Since the default value for notification_driver is noop
we just removed it.

Change-Id: I1eab3d4f7b0ccfe428a22be4d5553f6503ddf0aa
Merge "Remove notifier_strategy config from glance-api"