soc: nrf9160: Remove unsupported Kconfig option GPIO_AS_PINRESET

In nRF9160 the reset pin is a dedicated one, it cannot be configured
as a regular GPIO pin, so this option should not be presented to users
building for this SoC, to not generate confusion.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>