API endpoints DELETE /groups/:id and /v3/groups/:id executes more than 100 SQL queries

This controller executes more than 100 SQL queries, the maximum number of queries allowed as per MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16466. The number of queries should be reduced to less than 100 and the corresponding whitelisted sections of code (Gitlab::QueryLimiting.whitelist('...')) should be removed.

When destroying a group, number of queries is dependent on number of resources inside group.