ref: Fix missing votes for `DeleteRefs()` RPC

Production systems currently report that 20% of all RPCs which do not perform any votes is the DeleteRefs() RPC. A first guess is that this may happen in case a user requests deletions where there are no refs to be deleted, and that guess would be correct.

But in fact, none of the RPC calls will ever cast any votes: because of 99f0c28c (hook: Fix voting on pushes which delete packed references, 2021-02-15), we're forced to filter out any reference-transactions which consist solely of force deletions. And that's what DeleteRefs() does: it's force-deleting all refs which are requested for deletion, and thus the reference-transaction hook filters out the votes.

Fix the issue by doing a manual vote on all refs which are about to be deleted.