Remove now unnecessary `additional_rules!` method from GroupPolicy

The intention was for this to be used to override group policy in
EE. This led to all sort of strange problems in practice - for
example, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1025
needed to override `owner` permissions, but `additional_rules!`
passed `master` only. The better solution is to use an EE
module to override the policy. Removing this now defunct method.

app/policies/group_policy.rb

@@ -33,8 +33,6 @@ class GroupPolicy < BasePolicy
     if globally_viewable && @subject.request_access_enabled && !member
       can! :request_access
     end
-
-    additional_rules!(master)
   end
 
   def can_read_group?
@@ -45,8 +43,4 @@ class GroupPolicy < BasePolicy
 
     GroupProjectsFinder.new(@subject).execute(@user).any?
   end
-
-  def additional_rules!(master)
-    # This is meant to be overriden in EE
-  end
 end