selftests: gpio: remove obsolete gpio-mockup-chardev.c

GPIO selftests have changed to new gpio-mockup-cdev helper, so remove
old gpio-mockup-chardev helper.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>