net: lwm2m: Remove .well-known/core handling

`.well-known/core` resource is described by the CoAP RFC as an optional
method of resource discovery. The LwM2M specification though makes no
mentionon about this mechanism and provides an alternative method of
resource discovery instead (Device Management Discover, sec 5.4.2, and
Bootstrap Discovery, sec 5.2.7.3).

Since LwM2M does not require to implement `.well-known/core` resource
and it complicates the existing Discovery mechanism (and likely cause a
security concern) remove its handling from the LwM2M implementation.

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