Contest deletion fails if it contains a write-in candidate

Deleting a contest involving a write-in can fail because there would effectively be multiple attempts to delete the same write-in candidate: in DeleteContestDecisions(), DeleteContestResults(), and DeleteContestWriteins().

    // Delete results, decisions, contestants, and write-ins
    DeleteContestResults(*this, groupId, contestId);
    	// ReleaseWritein()
    		// conditional writeIns.erase(contestant);
    DeleteContestDecisions(*this, groupId, contestId);
    	// ReleaseWritein()
    		// conditional writeIns.erase(contestant);
    DeleteContestContestants(*this, groupId, contestId);
    DeleteContestWriteIns(*this, groupId, contestId);