drivers: net: kconfig: Remove unused SLIP_MTU symbol
drivers: net: kconfig: Remove unused SLIP_MTU symbol

Added in commit 184e251fdb ("slip: Add driver for host to qemu
connectivity"), then moved to drivers/net/Kconfig in commit 0612651deb
("drivers: slip: Consolidate under drivers/net/"). Never used.

Found with a script.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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