net: lwm2m: Deprecate LWM2M_ENGINE_MESSAGE_HEADER_SIZE

Kconfig value LWM2M_ENGINE_MESSAGE_HEADER_SIZE added
an extra headroom for CoAP packet sizes so that
if CoAP Block-Wise transfer block-size is configured
to be same as LWM2M_COAP_MAX_MSG_SIZE, the full
payload block would usually fit to the datagram.

This causes too much confusion to be usable.
CoAP headers and options vary on sizes, and there is
no runtime limitations that we should check the
header size against.

Only real limitation is the CoAP packet size, which
must fit into the UDP datagram with typical DTLS headers.

Only limitation for CoAP block-size then is that
it must fit into the CoAP message with all the headers
and options. But as the option sizes, like path, vary,
it must be checked runtime.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>