mfd: db8500-prcmu: Remove dead code for a non-existing config

The config DBX500_PRCMU_QOS_POWER was never introduced in the kernel
repository. So, the ifdef in  ./include/linux/mfd/dbx500-prcmu.h was never
effective.

Remove these dead function prototypes.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211227064839.21405-1-lukas.bulwahn@gmail.com