MAINTAINERS: remove self from GPIO maintainers

I have not been able to dedicate time to the GPIO subsystem since quite
some time, and I don't see the situation improving in the near future.
Update the maintainers list to reflect this unfortunate fact.
Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -5667,7 +5667,6 @@ F:	tools/testing/selftests/gpio/
 
 GPIO SUBSYSTEM
 M:	Linus Walleij <linus.walleij@linaro.org>
-M:	Alexandre Courbot <gnurou@gmail.com>
 L:	linux-gpio@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git
 S:	Maintained