Remove unnecessary final modifier in PlanExecutor.loadExecutorClass method

Remove unnecessary final modifier in PlanExecutor.loadExecutorClass private static method because private means you can not override this method.

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #163 from hsaputra/remove_unneeded_final_privatetatsic_planexecutor and squashes the following commits:

fe7afc7 [Henry Saputra] Remove unnecessary final modifier in PlanExecutor's private static method.