[IOT-2465] Fix DELETE request impl for cred

[IOT-2465] Fix DELETE request impl for cred

Verified using CTT 1.5.11 test CT1.7.9.3
DELETE delete all credentials return PASSED

Change-Id: Ie52c6af9fb9205a650b766acf0084e0ac2dcdcf1
Signed-off-by: Andrii Shtompel <a.shtompel@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21403
Reviewed-by: Aleksey Volkov <a.volkov@samsung.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Nathan Heldt-Sheller <nathan.heldt-sheller@intel.com>
Reviewed-by: Oleksii Beketov <ol.beketov@samsung.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>