API GET /groups/:id/milestones/:milestone_id/issues executes more than 100 SQL queries

Description

This API endpoint 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.

Impact

ToDo: Describe the impacts of the code improvements, such as metrics or screenshot before and after the associated MR(s) were merged.