power / PM: Eliminate CONFIG_PM_RUNTIME

After commit b2b49ccb (PM: Kconfig: Set PM_RUNTIME if PM_SLEEP is
selected) PM_RUNTIME is always set if PM is set, so #ifdef blocks
depending on CONFIG_PM_RUNTIME within #ifdef blocks depending on
CONFIG_PM may be dropped now.

Do that in drivers/power/pm2301_charger.c.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>