boards: silabs: Remove nonfunctional wakeup trigger

Remove wakeup trigger from Series 2 boards. The implementation
seems to configure a GPIO to be driven low on boot, which has
nothing to do with wakeup. In order to actually configure a
wakeup GPIO, the GPIO_INT_WAKEUP flag would need to be used when
registering an interrupt on the pin. However, this should not
be the responsibility of the board. It is an application
concern, so remove it from the boards.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>