Prevent warning about already defined constant.

git-svn-id: http://svn.redmine.org/redmine/trunk@16345 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/tasks/redmine.rake

       Target.connection.reset_pk_sequence!(table_name) if Target.primary_key
       target_count = Target.count
       abort "Some records were not migrated" unless source_count == target_count
+
+      Object.send(:remove_const, :Target)
+      Object.send(:remove_const, :Source)
     end
   end