drivers: eth: gmac: kconfig: remove unused kconfig symbol
drivers: eth: gmac: kconfig: remove unused kconfig symbol

The feature that used this symbol was removed in 18b07e09e000.
This is just a cleanup commit that removes the unused symbol.

Signed-off-by: Tomasz Gorochowik <265c7c25eca9aa9127910457b0aea7e793d329f1@antmicro.com>

finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr