[FIX] ir_model: Uninstall with orphan external ids of ir.model.fields

On uninstalling, if external ids are associated to an unexisting field, we do not try to delete the field (as it doesnt exist) and we remove the orphan external ids
Merge branch 'master' of bzr::lp:~openerp/openobject-server/saas-2 into ocb-saas-2