There's no need to vote to delete a closed proposal to have it removed.
As part of the routine maintenance of Area 51, proposals that have ABSOLUTELY NO ACTIVITY for a period of 30 days are subject to deletion. That means absolutely NO activity; no new questions, no comments, no voting, nothing. If a closed proposal gets no further activity, it will be deleted as part of that culling process.
Area 51 doesn't really need to have closed proposal hang around for future reference in the same way as Stack Overflow/Stack Exchange questions. The only reason to keep a closed proposal around is to give it a chance to be modified and possibly reopened. Barring that, proposals will be deleted automatically when it receives no further activity as part of the normal maintenance of Area 51.