net: lwm2m: do not expose security object

According to LwM2M specification 5.2.7.3 Bootstrap DISOCVER,
security object is only reported to the bootstrap server.

Correct the behavior to (1) report server object to the server
(2) do not report security object at registration time

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>