No option, no. Deal with it. :) You can, however, still use the petbar if you prefer that, because it will not be hidden.
However i fail to see the point why anyone wouldn't want to use the mainbar, specially in Teron fight, pressing 3-5 is way easier then using the petbar shortcuts
I hate it. I always broke MC by shielding myself, which I cannot do any more because shield is on my main action bar. Please add an option!