Use the coreMQTT Agent library component provided by
the Open IoT SDK instead of fetching it in Total
Solutions.

The Open IoT SDK component also provides a subscription
manager as well as an agent command processor. Therefore
the implementation of these modules is also removed from
Total Solutions as the OTA application now uses the
IOTSDK implementation.

Signed-off-by: Jack Styles <jack.styles@arm.com>