power: supply: core: unexport power_supply_property_is_writeable()

Since commit ("power: supply: Drop use_cnt check from power_supply_property_is_writeable()"),
this function does not check use_cnt anymore, making it unsuitable for
general usage. As it is only used by the psy core anyways, remove it
from the public header and unexport it to avoid misusage.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241005-power-supply-cleanups-v1-2-45303b2d0a4d@weissschuh.net


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>