Delete Client callback when receive observe cancel response.

Delete Client callback when receive observe cancel response.

Bug: https://jira.iotivity.org/browse/IOT-1317
Change-Id: I4e5ef2a1c848b963c036efe4ddf73dfd76b2b262
Signed-off-by: Ziran Sun <ziran.sun@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12055
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
(cherry picked from commit 888bc96303a22443ab4d75e225803268267956b0)
Reviewed-on: https://gerrit.iotivity.org/gerrit/12213
Reviewed-by: Habib Virji <habib.virji@samsung.com>