[FIX] Sync purchase orders on requistition unlink/cancel

Port of https://code.launchpad.net/~camptocamp/openobject-addons/7.0-fix_1220716-rha+srt+afe, as the remaining part of the failed merge of http://bazaar.launchpad.net/~ocb/ocb-addons/7.0/revision/9628 during the lp@gh migration (other changes are included in the upstream commit http://bazaar.launchpad.net/~openerp/openobject-addons/7.0/revision/9651.