Remove IotTaskPool_CreateRecyclableSystemJob() and IotTaskPool_ScheduleSystemJob...

Remove IotTaskPool_CreateRecyclableSystemJob() and IotTaskPool_ScheduleSystemJob() again, which were intended to be alternative APIs that only access the system task pool, and instead update IotTaskPool_CreateRecyclableJob() and IotTaskPool_ScheduleJob() to allow the parameter used to pass in the task pool handle to be NULL if the system task pool is the only one available.
Update the task pool demo app to include a lot more functionality.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2674 1d2547de-c912-0410-9cb9-b8ca96c0e9e2