IOT-2539 Removed unused-function

IOT-2539 Removed unused-function

The CompareCredential function is unused. Any code
that is unused is not maintained. This removes the
unused function. If the CompareCredential is needed
in the future it can be restored by reverting this
commit.

Bug: https://jira.iotivity.org/browse/IOT-2539
Change-Id: Ib5c1fee9a2bd90db3b55066a6e3b057760199b68
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21747
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Mats Wichmann <mats@linux.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>