net: lwm2m: remove "path" from object and resource instances

The path member of the object instance and resource instance structures
can easily be removed to save several bytes per instance over the entire
LwM2M subsystem.  So let's remove it.

Example savings when building for nrf52_blenano:
SRAM usage before patch: 37952 B
SRAM usage after patch: 36576 B

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>