net: lwm2m: Remove obsolete LWM2M_IPSO_TIMESTAMP_EXTENSIONS option

All IPSO objects that utilized the non-standard timestamp extension now
implement their model in version 1.1, which contains the Timestamp
resource. As it's no longer needed to add timestamp in a non-standard
way, simply remove LWM2M_IPSO_TIMESTAMP_EXTENSIONS option.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>