pinctrl: lantiq: Remove unsued declaration ltq_pinctrl_unregister()

Commit 3f8c50c9 ("OF: pinctrl: MIPS: lantiq: implement lantiq/xway pinctrl support")
declared but never implemented it, so can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230825092706.14680-1-yuehaibing@huawei.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>