notifier: remove notifier_strategy compat support

The notifier_strategy option is deprecated since we merged commit
I8cd84772bc5867e06b2a50ed7e15b9e86f0b94ad during the Icehouse cycle.

Assuming the usual single cycle deprecation period, this can now be
removed.

DocImpact: the notifier_strategy has been removed in favor of the
notification_driver and related oslo.messaging options.

Change-Id: I3ab0133b697a820536e5da84e30b078915a29f6c
Merge "notifier: remove notifier_strategy compat support"