Delete kernel created task in vTaskEndScheduler (#962)

Delete kernel created task in vTaskEndScheduler (#962)

* Update vTaskDelete() to delete a task directly when scheduler is stopped instead of putting it on the xTasksWaitingTermination list.
* Delete the idle tasks and timer task in vTaskEndScheduler().
* Reclaim resources for all the tasks on the xTasksWaitingTermination list in vTaskEndScheduler().
* Update POSIX to no longer delete FreeRTOS tasks in the port.

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Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>
Co-authored-by: Gaurav Aggarwal <aggarg@amazon.com>