mfd: ab8500-sysctrl: Drop ab8500_restart

ab8500_restart is not called from anywhere in the kernel, so drop it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>