Past challenges: NumOfSubmissions for design challenges includes checkpoint submissions

[SathyaJayabal]

In some cases(past challenges) the number of submissions (numOfSubmissions) returned includes the checkpoint submissions. So when the frontend adds numOfSubmissions + numOfCheckpointSubmissions, it gives the wrong number. Example: https://beta-community-app.topcoder.com/challenges/63ed01ee-c019-4c31-bcd5-9096a78929ef https://www.topcoder.com/challenges/30132448