Remove unused by_official_level scope from Proposal

The "by_official_level" scope in Proposal is no longer used anywhere in
the code. Its last use was removed in commit 9f1f912d84 ("Remove
official level filter from advanced search").

app/models/concerns/filterable.rb

@@ -2,10 +2,7 @@ module Filterable
   extend ActiveSupport::Concern
 
   included do
-    scope :by_date_range,     ->(date_range) { where(created_at: date_range) }
-    scope :by_official_level, ->(official_level) do
-      where(users: { official_level: official_level }).joins(:author)
-    end
+    scope :by_date_range, ->(date_range) { where(created_at: date_range) }
   end
 
   class_methods do