Fix the pending handover request can't be removed in TransportManager

After retriving a duplicate HO request, the TNF doesn't inform TransportManager
completed, so that TransportManager still holds the pending HO request. Thus,
the further HO request won't be continued to handle.

To fix it, send the callback with success to TransportManager.

Change-Id: Ib4796099adc3646fcfccf8c5a84f91437d068120
CRs-Fixed: 2625814