soc: samsung: pm_domains: drop unused is_off field

The 'is_off' member of internal state structure 'exynos_pm_domain' is
not used anymore.

Fixes: 2ed5f236 ("ARM: EXYNOS: Detect power domain state on registration from DT")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211008075253.67961-1-krzysztof.kozlowski@canonical.com