Remove the unused, undocumented SQLite3Adapter#supports_count_distinct?

This has returned true since 3cc9b5f1, and is not used internally.

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -189,11 +189,6 @@ module ActiveRecord
         @statements.clear
       end
 
-      # Returns true
-      def supports_count_distinct? #:nodoc:
-        true
-      end
-
       def supports_index_sort_order?
         true
       end