Change observe option header behaviour to be in line with spec

Change observe option header behaviour to be in line with spec
at the following points:

1. If the resource can't register the client as observer,
or the resource does not support observe, then the resource
sends back normal GET Response with NO observe option field.

2. Upon successfully removing the client from observer list,
the resource sends back a normal GET response with NO observe
option field

3. This patch also reverted https://gerrit.iotivity.org/gerrit/#/c/8901/
after discussed with the author to be inline with spec.

JIRA issue: https://jira.iotivity.org/browse/IOT-1122

Change-Id: I36510baa69a8ef3be9e0d9aef3fbd40d31488b0f
Signed-off-by: Ziran Sun <ziran.sun@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/8815
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Mushfiqul Islam <i.mushfiq@samsung.com>