Update privacy.thirdparty.isolate preference in Torbutton

privacy.thirdparty.isolate is gone with the switch to ESR52, yet we still try to use it in Torbutton. We should get rid of the preferences in torbutton_update_thirdparty_prefs() that we don't need anymore and adapt privacy.thirdparty.isolate for the remaining ones (it is privacy.firstparty.isolate now).