mfd: max77686: Remove unused extern declarations

mfd: max77686: Remove unused extern declarations

max77686_irq_init() and max77686_irq_exit() are not used since
commit 6f1c1e71d933 ("mfd: max77686: Convert to use regmap_irq").
And max77686_irq_resume() never be implemented since introduced in
commit dae8a969d512 ("mfd: Add Maxim 77686 driver").

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20230728132709.27052-1-yuehaibing@huawei.com
Signed-off-by: Lee Jones <lee@kernel.org>