samples: net: Fix coap-server sample

If client removes observer then it reply with RESET message for
further observer notifications. That means coap-server should stop
sending further notifications to the client.

Current sample unref the packet as soon as it finds pending message
and doesn't bother about message type.

Fixes: #6534

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>