csdk: Fixed compiler warnings in ocstack.c

csdk: Fixed compiler warnings in ocstack.c

-Wformat= warning fixed by changing the ssanf %lld flag to SCNd64
-Wunused-function fixed by removing the OCSetNetworkMonitorHandler
 function. Since the function is not used it is also not maintained
 it is best to remove the code.

Change-Id: Ib82e6e77a561ac8713cc77bdecc44a5ce42cefff
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17813
Reviewed-by: Mike Fenelon <mike.fenelon@microsoft.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>