Remove unnecessary check from Groups#AutocompleteService

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8632/diffs introduced an unnecessary check when performing auto complete for milestones in epics.

It checks if user can read current group parents which is not needed. When user can read
group he also can read its predecessors.

TODO

Remove ParentGroupsFinder if it is not being used on another place yet and use group.self_and_ancestors.

Edited Feb 25, 2019 by Felipe Cardozo