Remove IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ) from MQTT code

Remove IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ) from MQTT code

The cut down version of the task pool has only one task pool, namely,
system task pool. All the task pool API functions accept NULL as a
valid parameter for IotTaskPool_t and use the system task pool when
NULL is passed for the system task pool.

IOT_SYSTEM_TASKPOOL is defined to NULL to use system task pool and
therefore the above assert is no longer valid.