ironic: remove notifier_strategy option

This has never exited in Ironic, and it does not even uses notification.

Change-Id: I4a3d386116561d9a22d650f123df1aae5ed9849e
Merge "ironic: remove notifier_strategy option"
Merge commit 'refs/changes/99/70999/4' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch