boot: zephyr: Remove legacy serial recovery GPIO configuration

Change removes the legacy configuration. The legacy configuration
became problematic, because GPIO DTS nodes no longer support labels
that were used to identify nodes in MCUboot. Therefore we need to
use GPIO DTS node name with the legacy approach.

The GPIO should be configured by board's DTS, which is simpler.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>