move ability check from service class to view

app/views/search/_category.html.haml

@@ -84,6 +84,7 @@
           = _("Milestones")
           %span.badge.badge-pill
             = limited_count(@search_results.limited_milestones_count)
+      - if can?(current_user, :read_users_list)
         %li{ class: active_when(@scope == 'users') }
           = link_to search_filter_path(scope: 'users') do
             Users

lib/gitlab/search_results.rb

@@ -138,8 +138,6 @@ module Gitlab
     # rubocop: enable CodeReuse/ActiveRecord
 
     def users
-      return [] unless Ability.allowed?(current_user, :read_users_list)
-
       UsersFinder.new(current_user, search: query).execute
     end
 

spec/lib/gitlab/search_results_spec.rb

@@ -123,10 +123,6 @@ describe Gitlab::SearchResults do
     end
 
     describe '#users' do
-      it 'returns an empty array when the current_user is not allowed to read users list' do
-        expect(results.objects('users')).to be_empty
-      end
-
       it 'calls the UsersFinder' do
         expect(UsersFinder).to receive(:new).with(user, search: 'foo').and_call_original