Remove unused project model fields

- Remove `bookmark` field which we don't use anymore https://github.com/topcoder-platform/tc-project-service/issues/471
- Remove `challengeEligibility` as it looks like we've never used it https://github.com/appirio-tech/connect-app/issues/4270#issuecomment-765344461 and we already introducing a new field for `groups` https://github.com/topcoder-platform/tc-project-service/issues/616