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therefore the above assert is no longer valid.

FreeRTOS-Plus/Source/FreeRTOS-IoT-Libraries/c_sdk/standard/mqtt/src/iot_mqtt_operation.c

@@ -656,7 +656,7 @@ void _IotMqtt_ProcessKeepAlive( IotTaskPool_t pTaskPool,
     _mqttConnection_t * pMqttConnection = ( _mqttConnection_t * ) pContext;
 
     /* Check parameters. */
-    IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL );
+    /* IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ); */
     IotMqtt_Assert( pKeepAliveJob == pMqttConnection->keepAliveJob );
 
     /* Check that keep-alive interval is valid. The MQTT spec states its maximum
@@ -786,7 +786,7 @@ void _IotMqtt_ProcessIncomingPublish( IotTaskPool_t pTaskPool,
      * are disabled. */
     ( void ) pTaskPool;
     ( void ) pPublishJob;
-    IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL );
+    /* IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ); */
     IotMqtt_Assert( pOperation->incomingPublish == true );
     IotMqtt_Assert( pPublishJob == pOperation->job );
 
@@ -839,7 +839,7 @@ void _IotMqtt_ProcessSend( IotTaskPool_t pTaskPool,
      * are disabled. */
     ( void ) pTaskPool;
     ( void ) pSendJob;
-    IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL );
+    /* IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ); */
     IotMqtt_Assert( pSendJob == pOperation->job );
 
     /* The given operation must have an allocated packet and be waiting for a status. */
@@ -1020,7 +1020,7 @@ void _IotMqtt_ProcessCompletedOperation( IotTaskPool_t pTaskPool,
      * are disabled. */
     ( void ) pTaskPool;
     ( void ) pOperationJob;
-    IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL );
+    /* IotMqtt_Assert( pTaskPool == IOT_SYSTEM_TASKPOOL ); */
     IotMqtt_Assert( pOperationJob == pOperation->job );
 
     /* The operation's callback function and status must be set. */