mfd: ab8500: Remove unused extern declarations

commit d28f1db8 ("mfd: Remove confusing ab8500-i2c file and merge into ab8500-core")
left behind this.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20230728132439.31568-1-yuehaibing@huawei.com


Signed-off-by: Lee Jones <lee@kernel.org>