Remove handling of this property from code.

Note that wl->power_gpio is still needed in
the header file for SPI mode (N900).

Suggested by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>