application model: drop unused test_connection method.

app/models/application_record.rb

@@ -418,13 +418,6 @@ class ApplicationRecord < ActiveRecord::Base
       def columns(*params)
         super.reject {|x| x.sql_type == "tsvector"}
       end
-
-      def test_connection
-        limit(1).select(:id)
-        return true
-      rescue PG::Error
-        return false
-      end
     end
   end