Remove `User#projects_sorted_by_activity`

Its last usage has been removed at https://gitlab.com/mehlah/gitlab-ce/commit/9df6f7bfad#e6ff2c1091173fd03646752d4e30891f66d33db3_46_48
The method can still be useful but should be renamed `authorized_projects_sorted_by_activity`
for better clarity.

app/models/user.rb

@@ -664,10 +664,6 @@ class User < ActiveRecord::Base
     end
   end
 
-  def projects_sorted_by_activity
-    authorized_projects.sorted_by_activity
-  end
-
   def several_namespaces?
     owned_groups.any? || masters_groups.any?
   end