net: lwm2m: remove auto select of COAP_EXTENDED_OPTIONS_LEN config

When moving to the new CoAP API, I thought we would need to parse
incoming option values longer than 12 characters.

This hasn't proven to be true, so let's remove the auto-selection of
this config.  If needed user can set this option later.

Signed-off-by: Michael Scott <michael.scott@linaro.org>