tests uart/uart_pm: Fix kconfig warning

The option CONFIG_NATIVE_UART_0_ON_STDINOUT
is only usable for the native_sim/posix(_64) targets,
but this test is only allowed for the nrf52840dk_nrf52840.
Building with this option set causes a kconfig warning.
Let's just remove it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>