net: lwm2m: remove unused OBJ_FIELD_MULTI_DATA macro
net: lwm2m: remove unused OBJ_FIELD_MULTI_DATA macro

The use-case for this macro was removed during optional resource
changes.

Signed-off-by: Michael Scott <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@opensourcefoundries.com>

kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,mbolivar/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyriot/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyriot/zephyr,Vudentz/zephyr,galak/zephyr,punitvara/zephyr,kraj/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,zephyriot/zephyr,punitvara/zephyr,zephyriot/zephyr,punitvara/zephyr,explora26/zephyr,galak/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,mbolivar/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,mbolivar/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,punitvara/zephyr