Remove unused method

This method is not being called anywhere in our code and a GitHub search
show it is not being used in any project.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -157,11 +157,6 @@ def supports_primary_key?
         false
       end
 
-      # Does this adapter support using DISTINCT within COUNT?
-      def supports_count_distinct?
-        true
-      end
-
       # Does this adapter support DDL rollbacks in transactions? That is, would
       # CREATE TABLE or ALTER TABLE get rolled back by a transaction?
       def supports_ddl_transactions?