Bug 1354625 - Use mozilla-unified for single_locale try builds upstream. r=aki, a=NPOTB

I think I initially defined these and I think I know why I used
mozilla-central (it had to do with try not advertising bundles and
mozilla-unified being generaldelta when other repos were not). Those
reasons are no longer valid and we should be using mozilla-unified
everywhere.

MozReview-Commit-ID: CFaZspU6A5M