Remove unused external_source_visualization methods

That method is only used from the
VisualizationFactory#create_canonical_visualization which is only
invoked on Carto::UserTable models (via an after_create hook, BTW).

app/models/table.rb

@@ -1253,10 +1253,6 @@ class Table
     "#{prefix}#{name}_#{column}"
   end
 
-  def external_source_visualization
-    @user_table.try(:external_source_visualization)
-  end
-
   def previous_privacy
     # INFO: @user_table.initial_value(:privacy) weirdly returns incorrect value so using changes index instead
     privacy_changed? ? @user_table.privacy_was : nil

app/models/table/user_table.rb

@@ -298,10 +298,6 @@ class UserTable < Sequel::Model
     service.actual_row_count
   end
 
-  def external_source_visualization
-    data_import.try(:external_data_imports).try(:first).try(:external_source).try(:visualization)
-  end
-
   def table_visualization
     @table_visualization ||= map_id && CartoDB::Visualization::Collection.new.fetch(
       map_id: map_id,