samples: lwm2m_client: Rework to not define generated macros

Rather than redefining a macro which might be generated we should create
a new define name that should get used in the code.  Replace redefining
DT_ALIAS_LED0_GPIOS_* and just use the LED_GPIO_* defines that already
exist.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>