Bluetooth: OTC - do not register L2CAP server

Remove L2CAP server registration.  The object transfer client does not
need to register an L2CAP server.

OTS spec: "It is always the responsibility of the Client to open the
Object Transfer Channel, regardless of the direction of transfer, and
is never the responsibility of the Server."

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>