There is a very simple solution to this. Add an item to the agenda that you no longer require the attendance of the strata manager. Let everyone express their feelings and then go with the majority. But if the chairman isn’t prepared to chair, the next item after that should be vacation of the chair and election of a new chairman.
Seriously, though, a civilised, private discussion about why the SM is there and why the chairman doesn’t want to chair the meeting could get to a solution without anyone feeling ambushed or blind-sided.