gpiolib: of: Drop redundant check in of_mm_gpiochip_remove()

The callers never call the function with invalid pointer.
Moreover, compiler quite likely dropped that check anyway
because we use that pointer before the check.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>