net: lwm2m: simplify registration client

When designing the registration client for LwM2M, I understood
that the LwM2M Technical Specification allows for a multi-server
connection setup where the client makes several connections
to various LwM2M servers and allows each of them to manage
various aspects of the LwM2M client based on Access Controls.

However, the way I implemented it was not well thought out and
as we look forward to adding Bootstrap support, it needs a
do over.

Let's remove all of the code dedicated to handling multiple LwM2M
client connections.  This will simplify and reduce the code size
of the registration client considerably.

Later, once Bootstrap support has been added, we can implement
multi-server connections in a cleaner manner.

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