pmdomain: renesas: rcar-sysc: Remove rcar_sysc_nullify() helper

There are no more users left of the rcar_sysc_nullify() helper, so it
can be removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/ad61b09283cc8a9cf93a5ea9fffd1cb283b9db92.1713348705.git.geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>