gpio: pcie-idio-24: Prune superfluous license boilerplate

The SPDX License Identifier line renders the GPL boilerplate text
superfluous, so let's remove it.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>