net: lwm2m: Fix removed engine_observer_list usage

`engine_remove_observer_by_path()` was not updated during some recent
LwM2M observer changes, still using the `engine_observer_list` which got
moved into the `lwm2m_context` structure. Update the function to align
with these changes.

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