Use `descendant_project_members_with_inactive` in group_private_service.rb

What does this MR do and why?

Related issue: #417455 (closed)

Use descendant_project_members_with_inactive in group_private_service.rb

Fixes a cross-join betwee users and members table.

As User -> project_members association has requested_at: nil, it makes sense to use descendant_project_members_with_inactive method as this method has the non_request scope.

• Pass list of user_ids as a sub-query, instead of plucking. As members is gitlab_main_cell, and todos is likely to be also gitlab_main_cell, we can do this.

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