drivers: net: kconfig: Remove unused SLIP_MTU symbol

Added in commit 184e251f ("slip: Add driver for host to qemu
connectivity"), then moved to drivers/net/Kconfig in commit 0612651d
("drivers: slip: Consolidate under drivers/net/"). Never used.

Found with a script.

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